Fix counting and filtering on the dashboard page for issues

[wolfogre]

This PR actually 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 The incoming request has too many parameters. This server supports a maximum of 2100 parameters.  #26361
2. Fix repo selection, so it also fix Incorrect selection behavior at repo list , on the left side of the issues page. #26653
3. Keep keyword in filter links.

The first two are regressions of #26012.

After:

Screen.Recording.2023-08-22.at.17.51.28.mov

Thanks to @CaiCandong for helping with some tests.

[zjjhot]

Thank you very much. I have a suggestion.

Issues that have existed in history can be displayed in the repo list (even if this issue module is prohibited from accessing, it is reasonable to exist, and I can always see records of historical existence, which is fine).

However, those that have never been issued (count=0) should not be displayed in the repo list, especially those repos that disabled by issue module.

[wolfogre]

Issues that have existed in history can be displayed in the repo list (even if this issue module is prohibited from accessing, it is reasonable to exist, and I can always see records of historical existence, which is fine).

Not sure, maybe another PR.

However, those that have never been issued (count=0) should not be displayed in the repo list, especially those repos that disabled by issue module.

Done in e56c59a

[CaiCandong]

	for k, v := range issueCountByRepo {
		if _, ok := selectedReposMap[k]; !ok && v == 0 {
			delete(issueCountByRepo, k)
		}
	}

Maybe we can use https://pkg.go.dev/maps@go1.21.0#DeleteFunc

[wolfogre]

Maybe we can use pkg.go.dev/maps@go1.21.0#DeleteFunc

Not now, Gitea is still using go1.20:

gitea/go.mod

Line 3 in 1432d4e

go 1.20