add multi-device sync (#2669)

* add basic multi-device-sync functions

* generate json

* add context.parse_sync_items()

* add context.execute_sync_items()

* piggyback sync-commands message, add body for human-readable part

* avoid double json renderings

* mimeparser parses incoming .json sync-files

* do not piggyback sync-files

* execute sync items

* return status of send_sync_msg()

* send sync messages as multipart/report

* add a per-item-timestamp and also allow adding other per-item-fields in the future

* if the self-chat does not exist, create it blocked/hidden

* create tokens closer to real qr-code needs

* respect bcc_self setting, add test for that

* sync qr code tokens after promoting groups

* send sync-messages only if an experimental switch is set

* trigger send_sync_msg() after sending messages and after creating/redraw/revive qr-code

* add DC_STR_* constants to deltachat.h

* adapt to refactored qr module as of #2729

* tweak test

* use SendSyncMsgs config name instead of SendExperimentalSyncMsgs - we can remove or rename the config nevertheless, but have the option to keep it without renaming

* tweak docs

* remove currently unused effective timestamp calculation

* clarify when send_sync_msg() is called

* make sure, sync-messages are encrypted and are sent by SELF

* tweak docs, fix typos

src/sql/migrations.rs

@@ -487,6 +487,16 @@ paramsv![]
         )
         .await?;
     }
+    if dbversion < 80 {
+        info!(context, "[migration] v80");
+        sql.execute_migration(
+            r#"CREATE TABLE multi_device_sync (
+id INTEGER PRIMARY KEY AUTOINCREMENT,
+item TEXT DEFAULT '');"#,
+            80,
+        )
+        .await?;
+    }
 
     Ok((
         recalc_fingerprints,