Fix downloading extra data when run from a different thread

The main loop that is initiated while the extra data is being downloaded
needs a different context to be the thread-default one, otherwise the
operations just hang when running from a different thread.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f