Say how much an LDIF re-import duplicated, without acting on it

The half of #223 that can move while the matching question is still open.

Mozilla's schema defines no UID, so the import invents one and a re-import
duplicates everything. Whether to guess an identity from a name and an address
instead is the reporter's call and he has not made it -- but the harm that was
actually reported was confusion rather than duplication: somebody imports a
file twice and cannot tell what happened.

So the import now counts how many of the entries look like contacts the book
already held, and says so in a second message. Every card is still imported.
Nothing is skipped and nothing is merged, which is the point: counting is a
different act from matching, and it takes no decision away from the person who
still owes us one.

The likeness key is name plus one address, and it is wrong in both directions
by design -- two colleagues sharing a name and an alias collapse, somebody
whose address changed since the last export looks like a stranger. That is
tolerable for a number on a toast and would not be tolerable for a merge,
which is exactly why the number is all it does.

The scan the vCard import already makes for UIDs now collects names and
addresses on the same request, so this costs no extra round trip. It is read
before anything is created, so a file that repeats a person twice counts as
two new cards rather than as a duplicate of itself.

If the answer comes back "match on name and email", the matching is written
and becomes a skip instead of a count.

Refs #223.

web/src/locales/nl.ts

@@ -1048,6 +1048,7 @@ export const catalog: Catalog = {
     "Nothing unread here": "Hier is niets ongelezen",
   },
   plurals: {
+    "{n} of them look like contacts you already had": { one: "{n} daarvan lijkt op een contact dat u al had", other: "{n} daarvan lijken op contacten die u al had" },
     "Your administrator changed {n} settings": { one: "Uw beheerder heeft {n} instelling gewijzigd", other: "Uw beheerder heeft {n} instellingen gewijzigd" },
     "Already here: {n} contacts, nothing imported": { one: "Al aanwezig: {n} contact, niets geïmporteerd", other: "Al aanwezig: {n} contacten, niets geïmporteerd" },
     "All {n} are already in your contacts": { one: "Staat al in uw contacten", other: "Alle {n} staan al in uw contacten" },