Backport #17136
There was a serious issue with the `gitea dump` command in 1.14.3-1.14.6 which led to corruption of the `config` field of the `repo_unit` table.
This PR adds a doctor command to attempt to fix the broken repo_units. Users affected by #16961 should run:
```
gitea doctor --fix --run fix-broken-repo-units
```
Fix#16961
Signed-off-by: Andrew Thornton <art27@cantab.net>
