accounts.json: support new "enabled" key

Specifying false will cause Mirage to ignore an account and not
log it in on startup.
Defaults to true when adding new accounts or if not present.

src/backend/user_files.py

@@ -190,6 +190,7 @@ class Accounts(JSONDataFile):
                 "homeserver": client.homeserver,
                 "token":      client.access_token,
                 "device_id":  client.device_id,
+                "enabled":    True,
             },
         })