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/zh-Hans.ts

@@ -641,6 +641,12 @@ export const catalog: Catalog = {
     "+{n} more": "还有 {n} 项",
 
     "Contacts": "联系人",
+
+    "Collected": "已收集",
+
+    "Save people you write to as contacts": "将您发信的联系人保存为联系人",
+
+    "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.": "您发信的对象如果还不是联系人，会被添加到“已收集”通讯录，在所有设备上都会作为建议显示。您自己的地址不会被添加。",
     "Contacts are not available": "联系人不可用",
     "New contact": "新建联系人",
     "Edit contact": "编辑联系人",