Add Tenants to Administration, and let an account be put in one

A tenant is a separate organisation on one server: its own people,
domains and limits, and an administrator who manages only what is in it.
It gets a section under Access, gated by sysTenantQuery and sysTenantGet,
with a notice on a server that does not report Enterprise, where anyone
inside a tenant is held to an ordinary user's permissions.

The panel edits the tenant's name, logo, role and limits. The logo is an
https address, drawn through the image proxy the strict image policy
requires, or an image data URL. Limits change one quotas/<name> pointer
each, so the four ihasmail does not offer keep their values, and an empty
field is no limit. The role is the most anyone inside can be allowed.

Stalwart keeps no list on a tenant -- each account, group, domain, list and
role names its own -- so what a tenant holds is counted with memberTenantId
queries and shown against its limits. Domains are added and taken out from
the tenant's panel, one memberTenantId change each; only a domain in no
tenant can be added, and its accounts stay where they are. Delete is offered
once every count reads zero.

A tenant does nothing until someone administers it, so the account panel
gains a Tenant choice for an administrator who can read tenants: an
Administrator inside a tenant administers that tenant. Nobody moves their
own account.

The mock has a tenant holding a domain and an administrator, a spare domain
to assign, memberTenantId filters on every query, and Stalwart's rule that
only an administrator outside every tenant may move things into one. A test
of taking a domain back out found that the mock's pointer handling dropped a
top-level null instead of storing it, so nothing had ever been cleared that
way; it stores null now, as the server reads it back.

Nothing about tenants has been written on a live server: production has
none. KNOWN-ISSUES says what was read from source.

Thirty-nine new strings and one plural, in all nine catalogues.

ROADMAP.md

@@ -8,7 +8,7 @@ the rest is here because the answer is "no", not "not yet".
 
 See [KNOWN-ISSUES.md](KNOWN-ISSUES.md) for what is built but worth knowing about.
 
-- **Administration beyond accounts, groups, mailing lists, roles and domains.** The Administration menu opens on a dashboard and manages accounts, groups, mailing lists, roles and domains today — see [FEATURES.md](FEATURES.md#administration). DNS and ACME providers and tenants are Stalwart registry objects behind the same permission model, and each is a section to add rather than a design to invent; so is switching a domain's DNS, DKIM or certificate management, which is shown but not yet changed from ihasmail. The dashboard reads a handful of numbers and stops there. Managing queues, reading logs and changing server settings are not planned: they are operating the server, which is Stalwart's own interface's job, not managing the people on it.
+- **More of Stalwart's directory in Administration.** The Administration menu opens on a dashboard and manages accounts, groups, mailing lists, tenants, roles and domains today — see [FEATURES.md](FEATURES.md#administration). DNS and ACME providers are Stalwart registry objects behind the same permission model, and each is a section to add rather than a design to invent; so is switching a domain's DNS, DKIM or certificate management, which is shown but not yet changed from ihasmail. The dashboard reads a handful of numbers and stops there. Managing queues, reading logs and changing server settings are not planned: they are operating the server, which is Stalwart's own interface's job, not managing the people on it.
 - **Sharing a mail folder.** Stalwart stores the share and never delivers it; see [KNOWN-ISSUES.md](KNOWN-ISSUES.md). Withdrawn until the server does something with it. Sharing files, calendars and address books is unaffected and works.
 - **A scheduling view of its own**, for asking "when is everyone free next week?" without an event in hand. The grid itself is built and lives in the event editor — a row per participant, steppable, and clickable to place the event — which is where the question gets asked while you are arranging something. What is not built is the same thing as a destination you can visit with nothing in progress. Came out of [#172](https://github.com/Coffey-Labs/ihasmail/issues/172), which asked for a separate view and is closed by the panel: the reasoning for putting it in the editor is that a separate surface can only ever tell you a time you then retype, whereas one beside the event can set it. It stays here rather than in the tracker because nobody has yet said they want to ask the question on its own.
 - **Per-message actions from the message list on a touchscreen.** Reply, Forward and compose-as-new are on the list row's context menu, which is a right-click — and holding a row on a phone starts selection instead, so none of them are reachable there. They are all available inside a thread, which is where the actions on a single message belong; what is missing is the shortcut from the list. Fixing it means deciding what a long press should do when it already means something, which is a bigger question than the actions themselves.