allow filtering issue queue by "series"

now that the new release system is deployed, there are going to be more and more different versions floating around as choices in various issue queues. we've always had this problem for core, and now it's going to be a problem for contribs, too. for example, in core, right now to see all the 5.x-related issues, you need something like:

http://drupal.org/project/issues?projects=3060&versions=94737,94702,9692...

as soon as we add beta2 you'd have to add another values for the versions= argument, and when we add an RC, yet another...

instead of manually updating bookmarks and issue queue links like this, it'd be slick to provide something like:

http://drupal.org/project/issues?projects=3060&series=5.x

we already have our happy taxonomy for "Drupal core compatibility", with values like "4.7.x", "5.x", etc. it should be a relatively simple INNER JOIN on {term_node} to get all the right versions options dynamically...