Make an event out of a message

Asked for in #167: a right-click on a mail that turns it into a calendar
entry, the way a bill or a task becomes a reminder. Nothing clever, and
deliberately so -- the subject becomes the title, the body becomes the
description, and the reader supplies the one thing the message cannot.

A due date is exactly that thing. "Due on the 14th" in an invoice is not
a date a parser could be trusted with, and a wrong guess quietly
scheduled is worse than no guess at all, so the editor opens on the next
half hour for an hour and the reader fixes it. Forward rather than now,
because a start time that has already passed by the time they press
Create is one more thing to correct.

The body is capped at 5000 characters. A newsletter is a message too,
and its whole body would be stored on the event, synced to every device,
and shown in a three-row textarea; what is worth keeping -- the amount,
the account, the address -- is near the top. The cut is marked, so a
truncated bill is not read as the whole of it.

One message only. The list menu acts on the selection everywhere else,
but there is no sensible event to make out of five mails, and the mobile
entry appears only when exactly one row is held.

The editor lives inside CalendarView and the reader is in the mail view
when they ask, so the draft waits in the calendar store until that view
mounts and takes it -- once, or it would reopen on every later visit.
It seeds a form rather than an event: the dialog still says New event
and still has to be pressed.

Called *Create event…* rather than "appointment", which is the word the
issue used: it opens the New event dialog, and each catalogue already
has its own settled noun for that -- Termin, événement, 日程.

Reachable three ways, since a phone has no right-click: the row context
menu, a message's ⋮, and the ⋮ of a held row on mobile. Hidden entirely
where the account has no calendar.

FEATURES.md

@@ -140,7 +140,7 @@ more attempt. A drag that is merely more sideways than not stays a scroll.
 - **Drag and drop** onto any folder in the tree, moving the selection or the
   row under the cursor.
 - **Context menu** on any row: reply, forward, archive, delete, spam, read/unread,
-  star, move to…, label…, and *Filter messages like this…*.
+  star, move to…, label…, *Filter messages like this…*, and *Create event…*.
 - **Snippets and avatars** are optional; the star is always in the row.
 - **Skeleton rows** while a page loads, rather than an empty pane.
 
@@ -382,7 +382,7 @@ nothing for anybody else.
 
 Created by clicking an empty slot or dragging across a range; a context menu on
 empty space offers a timed or all-day event at that moment, or *Go to day* /
-*Go to week*.
+*Go to week*. Also from a message — see *Create event…* below.
 
 The editor covers title, start and end (all-day or timed, with a time zone),
 calendar, location, meeting link, guests, description, reminders, repeat,
@@ -398,6 +398,10 @@ status (confirmed / tentative / cancelled), show-as (busy / free), visibility
   picker that predated them is gone; a colour comes from the category, or the
   calendar.)
 - **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.
 - **Popover** on click with the detail and quick actions; the editor on
   *Edit…*.