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/stock_str.rs

@@ -276,6 +276,15 @@ pub enum StockMessage {
     #[strum(props(fallback = "Download maximum available until %1$s"))]
     DownloadAvailability = 100,
 
+    #[strum(props(fallback = "Multi Device Synchronization"))]
+    SyncMsgSubject = 101,
+
+    #[strum(props(
+        fallback = "This message is used to synchronize data between your devices.\n\n\
+                    👉 If you see this message in Delta Chat, please update your Delta Chat apps on all devices."
+    ))]
+    SyncMsgBody = 102,
+
     #[strum(props(fallback = "Incoming Messages"))]
     IncomingMessages = 103,
 
@@ -624,6 +633,16 @@ pub(crate) async fn ac_setup_msg_body(context: &Context) -> String {
     translated(context, StockMessage::AcSetupMsgBody).await
 }
 
+/// Stock string: `Multi Device Synchronization`.
+pub(crate) async fn sync_msg_subject(context: &Context) -> String {
+    translated(context, StockMessage::SyncMsgSubject).await
+}
+
+/// Stock string: `This message is used to synchronize data betweeen your devices.`.
+pub(crate) async fn sync_msg_body(context: &Context) -> String {
+    translated(context, StockMessage::SyncMsgBody).await
+}
+
 /// Stock string: `Cannot login as \"%1$s\". Please check...`.
 pub(crate) async fn cannot_login(context: &Context, user: impl AsRef<str>) -> String {
     translated(context, StockMessage::CannotLogin)