Update a re-imported event rather than skipping it

Contacts and calendars disagreed on a re-import: a vCard or LDIF entry
whose identity a book already held overwrote the card there (#242, #274),
while an event whose UID a calendar held was counted and thrown away
(#222). The asymmetry was never decided -- it was where each half stopped.

Decided on #279: calendars update too, with two properties held back.
`participants` carries every attendee's accepted/declined and
`recurrenceOverrides` holds every "just this Wednesday" edit made here.
Both are decisions taken after the file was written, and a file that
mentions them at all describes them as they were at export, so writing
either one over would destroy work silently and return no error. A
corrected export now fixes the time, the title and the location, and
leaves who said yes alone. `uid` is held back with them: it is what the
two were matched on, so it is already equal.

The scan returns uid -> id rather than a set of UIDs, since updating
needs something to address, and creates and updates now share one
`maxObjectsInSet` budget the way contacts' `writeCards` does -- 300 new
and 300 changed batched separately would be two calls of 300, neither
over a ceiling of 500 and both refused. Counts become created/updated,
reported as the contacts import reports them.

Still no scheduling messages, on an update as much as on a create. That
is a real cost -- an event a re-import moves is moved here and nowhere
else -- and it is the lesser one: an import is not the place to start
mailing a room full of people who never asked for it.

Driven against the mock end to end: a second file with the same UID
updated the event in place, took the file's title, start and location,
and left an accepted RSVP and a per-occurrence override untouched even
though the file carried participants of its own.

web/src/locales/zh-Hans.ts

@@ -1387,8 +1387,7 @@ export const catalog: Catalog = {
     "Your administrator changed {n} settings": { other: "管理员更改了 {n} 项设置" },
     "Exported {n} events": { other: "已导出 {n} 个日程" },
     "Imported {n} events": { other: "已导入 {n} 个日程" },
-    "Already here: {n} events, nothing imported": { other: "已存在 {n} 个，未导入" },
-    "{n} were already here": { other: "{n} 个已存在" },
+    "Updated {n} events, nothing new": { other: "已更新 {n} 个日程，无新增" },
     /*
      * One form each, because Chinese has one. Intl.PluralRules returns `other`
      * for every number, so `one`, `few` and `many` would never be selected —