3905 Using a tag set with long tags causes challenge sign-up display problems

https://code.google.com/p/otwarchive/issues/detail?id=3905

If you use a tag set for your challenge, and your tag set has a long tag or two, things get ugly on the form. This makes them not get ugly.
This was using `tag_type` instead of `tag_name`, which meant it would say "Any Freeform" instead of "Any Additional Tag." I had to `singularize` it because we use "Additional Tags" instead of "Additional Tag" for the `tag_name` despite the fact fandom, relationship, and character are all singular by default.