Skip vCards on re-import that the address book already has

The contacts half of the rule that shipped for events, and only the half that
can be decided. A vCard carries a UID its author meant, so a card whose UID
this book already holds is that card, and re-importing an export left a second
copy of every one of them. Reported on #174 by the reporter's colleague, and
decided on #173: skip on a UID that is already here, import what arrives
without one, since nothing can be matched on an identity that is not there.

LDIF is deliberately untouched and now says so in the type. Mozilla's schema
defines no UID and the dn is not an identity outside the directory it came
from, so the import invents a UID that can never match one already present.
Guessing instead from a name and an address is the open question on #223, and
a guess that merges two people who share a name is worse than a duplicate
somebody can see and delete.

Both imports answer with the same shape, so a caller does not have to know
which one it called. LDIF's skipped is always 0, which is the honest number
rather than a missing field.

The UIDs are asked of the server rather than read from the cards in the store.
The store's copy is complete once the view has loaded, and importing does not
wait for a view.

Two callers, two messages. The contacts import reports both counts, as the
calendar import does: "Imported 3 contacts" over a file of two hundred reads
as a failure when the rest were already here. And a vCard attached to a
message -- usually one you have been sent before -- now says it is already in
your contacts rather than reporting that it added none.

Refs #223; the LDIF half stays open.

web/src/locales/de.ts

@@ -1078,6 +1078,8 @@ export const catalog: Catalog = {
     "Nothing unread here": "Hier ist nichts ungelesen",
   },
   plurals: {
+    "Already here: {n} contacts, nothing imported": { one: "Bereits vorhanden: {n} Kontakt, nichts importiert", other: "Bereits vorhanden: {n} Kontakte, nichts importiert" },
+    "All {n} are already in your contacts": { one: "Bereits in Ihren Kontakten", other: "Alle {n} sind bereits in Ihren Kontakten" },
     "Exported {n} events": { one: "{n} Termin exportiert", other: "{n} Termine exportiert" },
     "Imported {n} events": { one: "{n} Termin importiert", other: "{n} Termine importiert" },
     "Already here: {n} events, nothing imported": { one: "Bereits vorhanden: {n} Termin, nichts importiert", other: "Bereits vorhanden: {n} Termine, nichts importiert" },