Create doctor command to fix repo_units broken by dumps from 1.14.3-1.14.6 (#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>

models/repo_unit.go

@@ -220,3 +220,9 @@ func getUnitsByRepoID(e db.Engine, repoID int64) (units []*RepoUnit, err error)
 
 	return units, nil
 }
+
+// UpdateRepoUnit updates the provided repo unit
+func UpdateRepoUnit(unit *RepoUnit) error {
+	_, err := db.GetEngine(db.DefaultContext).ID(unit.ID).Update(unit)
+	return err
+}