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/store/settings.ts

@@ -206,6 +206,15 @@ export interface Settings {
    * dates nobody put there is a surprise rather than a feature.
    */
   birthdayCalendar: boolean;
+  /**
+   * Save everyone written to who is not a contact yet, in an address book of
+   * its own, so they are suggested on every device rather than only in the
+   * browser that sent the message. On by default, as mail clients do; the
+   * address book can be emptied or deleted like any other.
+   */
+  collectRecipients: boolean;
+  /** The address book collected recipients go to, once there is one. */
+  collectedBookId: string | null;
   /**
    * Calendars subscribed to by URL. The subscription is the setting; the
    * events themselves are fetched on demand and never stored, so this follows
@@ -361,6 +370,8 @@ export const DEFAULT_SETTINGS: Settings = {
   timeZone: null,
   labelsSidebar: true,
   birthdayCalendar: false,
+  collectRecipients: true,
+  collectedBookId: null,
   icalSubscriptions: [],
   listSortPreset: "newest",
   listSortLevels: [],