Backport #21033
In #21031 we have discovered that on very big tables postgres will use a
search involving the sort term in preference to the restrictive index.
Therefore we add another index for postgres and update the original migration.
Fix#21031
Signed-off-by: Andrew Thornton <art27@cantab.net>
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>