Add rel="nofollow" to issue filter links

The issue filter links should not be crawled by search engines, because
they they only filter results, and contain nothing new, yet, they put a
considerable load on the server.

To stop - well behaving - search engines from following these links, add
a `rel="nofollow"` property to them. The same property is already
present on the archive download links, and plenty of other places.

Fixes go-gitea#2361.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>