Invite the people the message was already between

Follow-up to #167: an event made from a mail now opens with the sender
and everyone it was addressed to already in the guest list, so a thread
becomes a meeting without retyping the room.

Two things are deliberately left out. The reader's own addresses, since
they are the organiser and an organiser among their own guests is an
invitation to your own appointment. And a blind copy, on a message the
reader sent: a guest list is visible to every guest, so promoting a Bcc
to a guest would tell the room about a copy the sender chose to hide.
That is not something a menu item may do quietly.

*Send invitation emails to guests* now starts off when the guests were
inherited rather than typed, and on everywhere else -- which is every
event whose guests somebody chose one at a time. The reason is the case
the issue opened with: a reminder made out of a bill carries the biller
and everyone else on the mail. Left on, the primary button reads Send
invites and the first press mails all of them an invitation to what was
meant as a note to self. The switch sits right there under the list and
says what it does, so inviting them is one deliberate click. Un-sending
is not.

FEATURES.md

@@ -400,8 +400,12 @@ status (confirmed / tentative / cancelled), show-as (busy / free), visibility
 - **Duplicate** an event from the context menu.
 - **Create event…** from a message, in its context menu and its ⋮ menu (and,
   on a phone, in the ⋮ of a held row). The subject becomes the title and the
-  body the description; the editor opens on the next half hour for an hour,
-  because when it happens is the one thing the message cannot say.
+  body the description; the sender and everyone the message was addressed to
+  become guests, minus your own addresses and never a blind copy. The editor
+  opens on the next half hour for an hour, because when it happens is the one
+  thing the message cannot say — and with *Send invitation emails* off, since
+  a guest list you inherited rather than typed should not mail itself on the
+  first press.
 - **Popover** on click with the detail and quick actions; the editor on
   *Edit…*.