Fix counting and filtering on the dashboard page for issues (#26657)
This PR has multiple parts, and I didn't split them because
it's not easy to test them separately since they are all about the
dashboard page for issues.
1. Support counting issues via indexer to fix #26361
2. Fix repo selection so it also fixes #26653
3. Keep keywords in filter links.
The first two are regressions of #26012.
After:
https://github.com/go-gitea/gitea/assets/9418365/71dfea7e-d9e2-42b6-851a-cc081435c946
Thanks to @CaiCandong for helping with some tests.
Jason Song