Add exposed filter to display and filter by available issue tags

The new issue tags are great. But there is one caveat: There is no list of available tags for a project - it is like a black hole: you tag some issues, but you have to know the tag to make any use of it. That is not only an major issue for users, but also for project maintainers.

What I imagine is a new exposed filter select list that contains all terms for a project, which have at least one issue assigned. Certainly, this filter would only be available for a single project (like the version filter).

That way,

- users could see all issues with a certain tag in a project, for example "release blocker".

- project maintainers are able to _maintain_ their issue tags, by re-/un-/assigning proper tags to issues.

- project maintainers are able to help site maintainers to remove orphan and needless terms by cleaning up their issue tags, so issue tags with 0 assigned issues can be deleted automatically.