Save the people you write to as contacts

The addresses written to were remembered only in the browser that sent
the message, as a list of recent recipients, so a new device or a cleared
browser suggested nobody. After each confirmed send, the recipients who
are not contacts yet are now saved on the server, in an address book of
their own called Collected, so they are suggested everywhere.

- Only addresses on no card in any address book, own or shared, are
  added, de-duplicated, and never the sender's own identities.
- The book is created on first use and remembered by id in the synced
  settings, so its name can be anything; a deleted one is replaced.
- Names are split as the contact editor does ("Smith, Jane").
- Settings > Calendar & contacts > Contacts has a switch, on by default,
  as mail clients do; the book can be emptied or deleted like any other.

New strings: 3, in all ten catalogs (1699 -> 1702).

web/src/locales/de.ts

@@ -651,6 +651,12 @@ export const catalog: Catalog = {
     "+{n} more": "+{n} weitere",
 
     "Contacts": "Kontakte",
+
+    "Collected": "Gesammelt",
+
+    "Save people you write to as contacts": "Personen, denen Sie schreiben, als Kontakte speichern",
+
+    "Everyone you send to who isn’t a contact yet is added to the Collected address book, so they’re suggested on every device. Your own addresses are never added.": "Alle, an die Sie senden und die noch keine Kontakte sind, werden dem Adressbuch „Gesammelt“ hinzugefügt und auf jedem Gerät vorgeschlagen. Ihre eigenen Adressen werden nie hinzugefügt.",
     "Contacts are not available": "Kontakte sind nicht verfügbar",
     "New contact": "Neuer Kontakt",
     "Edit contact": "Kontakt bearbeiten",