Fix more HTMLURL in templates (#22831)

Fix more HTMLURL in templates (#22831)

I haven't tested `runs_list.tmpl` but I think it could be right.

After this PR, besides the `<meta .. HTMLURL>` in html head, the only
explicit HTMLURL usage is in `pull_merge_instruction.tmpl`, which
doesn't affect users too much and it's difficult to fix at the moment.

There are still many usages of `AppUrl` in the templates (eg: the
package help manual), they are similar problems as the HTMLURL in
pull_merge_instruction, and they might be fixed together in the future.

Diff without space:
https://github.com/go-gitea/gitea/pull/22831/files?diff=unified&w=1