Skip events on re-import that the calendar already has

Importing an export twice left second copies of everything. The import has
kept the file's own UID since it was written -- inventing one only where an
event arrives without -- so what was needed to recognise an event that is
already here was there all along, and nothing looked at it.

Asked for on #173 after the reporter's colleague hit the duplication in
testing, and decided there: skip on a UID the calendar already holds, import
what arrives without one. An event with no UID is not one anything can match
to, and a softer match -- title and time, say -- guesses in both directions.

The UIDs are read once per import rather than once per event. CalendarEvent/
query does take a uid filter, which is what findByUid uses, but a file of two
thousand events would be two thousand queries. Read without expandRecurrences
so a weekly series is one event with one UID rather than one per occurrence,
and narrowed to the target calendar from calendarIds rather than through an
inCalendar filter this client has not confirmed the server supports.

Matching is per calendar. A UID is what makes an event the same event across
calendars, so the same event being in two of them is not a duplicate and the
second calendar still gets its copy.

importIcs now answers with both counts. "Imported 40 events" over a file of
240 reads as a failure when 200 of them were simply already there, and a
re-import of an unchanged file would otherwise report importing nothing at
all rather than saying everything was already here.

The three import toasts are translated in all nine catalogues while the
messages were being written -- the plural for the existing one had never been
added and was falling back to English.

Closes #222.

web/src/locales/fr.ts

@@ -1051,6 +1051,9 @@ export const catalog: Catalog = {
     "Nothing unread here": "Rien de non lu ici",
   },
   plurals: {
+    "Imported {n} events": { one: "{n} événement importé", other: "{n} événements importés" },
+    "Already here: {n} events, nothing imported": { one: "Déjà présent : {n} événement, rien d’importé", other: "Déjà présents : {n} événements, rien d’importé" },
+    "{n} were already here": { one: "{n} était déjà présent", other: "{n} étaient déjà présents" },
     "{n} messages": { one: "{n} message", other: "{n} messages" },
     "{n} selected": { one: "{n} sélectionné", other: "{n} sélectionnés" },
     "{n} conversations": { one: "{n} conversation", other: "{n} conversations" },