Import an iCal file into a calendar

An .ics reaches you by ways that are not your mailbox -- a ticketing
system a customer invited, a colleague's export, a booking confirmation
forwarded on -- and until now the only events ihasmail could take were the
ones attached to a message it had received.

The calendar's own menu now offers "Import iCAL file…", which files
everything in the file into that calendar. No global button: the issue is
right that this is not a frequent enough thing to earn one.

The parsing is the server's, through the same CalendarEvent/parse an
emailed invitation already goes through. An .ics is not a format worth
reimplementing in a browser, and Stalwart's reader handles what a
hand-rolled one would not.

Every event goes out in a single CalendarEvent/set. The round trips are
the smaller half of the reason: createEvent invalidates on the way out and
invalidating refetches every cached range, so a year of events imported
one at a time would refetch the calendar a few hundred times.

Nothing is mailed to anyone named in the file. Importing is filing
something you already have, and scheduling messages would be a surprise to
its participants.

The mock's parser read the whole file with one regex and returned one
event, which is all an invitation ever needed. It now reads per VEVENT, so
a multi-event file can be tested against it, and it invents an organiser
and an attendee only for events that carry a METHOD -- a plain export is
not addressed to anyone.

Closes #173

FEATURES.md

@@ -43,7 +43,7 @@ capability removes its feature rather than breaking the app.
 | `urn:ietf:params:jmap:vacationresponse` | Out of office | The Settings section hides |
 | `urn:ietf:params:jmap:sieve` | Filters, visual and raw | Filters hides |
 | `urn:ietf:params:jmap:contacts` (+`:parse`) | Contacts; vCard import | Contacts hides; import only needs `parse` |
-| `urn:ietf:params:jmap:calendars` (+`:parse`) | Calendar; iTIP invitations in mail | Calendar hides; invite cards do not render |
+| `urn:ietf:params:jmap:calendars` (+`:parse`) | Calendar; iTIP invitations in mail; iCal import | Calendar hides; invite cards and import need `parse` |
 | `urn:ietf:params:jmap:principals` | Directory lookup, sharing pickers | Sharing and directory autocomplete step aside |
 | `urn:ietf:params:jmap:principals:availability` | Free/busy when scheduling | Guests show no availability |
 | `urn:ietf:params:jmap:quota` | Storage bar under the folder list | The bar is not drawn |
@@ -378,6 +378,10 @@ Right-click your own to rename, recolour, share, stop sharing or delete;
 right-click one of someone else's to remove it from your view, which changes
 nothing for anybody else.
 
+- **iCal import** through `CalendarEvent/parse` (a file of any number of
+  events), from the calendar's own menu, into that calendar. The events are
+  filed rather than scheduled: no invitations go out to anyone named in them.
+
 ## Events
 
 Created by clicking an empty slot or dragging across a range; a context menu on