Backport #20158
Unforunately the previous PR #20035 created indices that were not helpful
for SQLite. This PR adjusts these after testing using the try.gitea.io db.
Fix #20129
Signed-off-by: Andrew Thornton <art27@cantab.net>
We have the `AppState` module now, it can store app related data easily. We do not need to create separate tables for each feature.
So the update checker can use `AppState` instead of a one-row dedicate table.
And the code of update checker is moved from `models` to `modules`.