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/context.rs

@@ -303,6 +303,7 @@ impl Context {
         let e2ee_enabled = self.get_config_int(Config::E2eeEnabled).await?;
         let mdns_enabled = self.get_config_int(Config::MdnsEnabled).await?;
         let bcc_self = self.get_config_int(Config::BccSelf).await?;
+        let send_sync_msgs = self.get_config_int(Config::SendSyncMsgs).await?;
 
         let prv_key_cnt = self
             .sql
@@ -392,6 +393,7 @@ impl Context {
             self.get_config_int(Config::KeyGenType).await?.to_string(),
         );
         res.insert("bcc_self", bcc_self.to_string());
+        res.insert("send_sync_msgs", send_sync_msgs.to_string());
         res.insert("private_key_count", prv_key_cnt.to_string());
         res.insert("public_key_count", pub_key_cnt.to_string());
         res.insert("fingerprint", fingerprint_str);