Prompt for Tag / Link Sets overlap where you are typing

Summary: When you are editing a Tag or Link Set, and you begin to type a name, it helpfully prompts you with existing values to choose from. Unhelpfully, the prompt comes up over where you are typing, so you can't see.

Justin: frankly, I'm astonished nobody's complained about this yet. Been driving me nuts...

Justin: the problem seems to have been putting a hard-coded span10 on the .mf_container, to give the Tag Set a consistent size; this caused its formatting to choke. I think I'm just going to declare that a bad idea, and instead encourage folks to use the {{spannn:...}} mechanism themselves instead.