Make offline presence to stop sync

Setting the presence of an account to offline
will make the client to end sync task and will
prevent messages from being sent.

Setting it to online again or any other presence
will start sync task again.

Left:
- Local echo to presence change
- UI Control to affect all members presence
- Block more requests to be sent (e.g. member
  actions)

src/backend/user_files.py

@@ -184,7 +184,7 @@ class Accounts(JSONDataFile):
 
         client   = self.backend.clients[user_id]
         saved    = await self.read()
-        presence = self.backend.models["accounts"][user_id].presence.value
+        account  = self.backend.models["accounts"][user_id]
 
         await self.write({
             **saved,
@@ -193,8 +193,10 @@ class Accounts(JSONDataFile):
                 "token":      client.access_token,
                 "device_id":  client.device_id,
                 "enabled":    True,
-                "presence":   presence or "online",
-                "order":      max([
+                "presence":   account.presence.value,
+
+                # Can account.order converge with any other saved value?
+                "order":      account.order if account.order >= 0 else max([
                     account.get("order", i)
                     for i, account in enumerate(saved.values())
                 ] or [-1]) + 1,
@@ -202,6 +204,24 @@ class Accounts(JSONDataFile):
         })
 
 
+    async def update(
+        self,
+        user_id:  str,
+        enabled:  Optional[str] = None,
+        presence: Optional[str] = None,
+        order:    Optional[int] = None,
+    ) -> None:
+        """Update existing account in the config and write to disk."""
+
+        saved = await self.read()
+
+        saved[user_id]["enabled"]  = enabled  or saved[user_id]["enabled"]
+        saved[user_id]["presence"] = presence or saved[user_id]["presence"]
+        saved[user_id]["order"]    = order    or saved[user_id]["order"]
+
+        await self.write({**saved})
+
+
     async def delete(self, user_id: str) -> None:
         """Delete an account from the config and write it on disk."""