Add category indicator to search box

[pferreir]

Since it's not clear when we are searching inside a category or globally (and people may want to do either), we could add an indicator in the style of that available in Google Docs. basically, we'd show a small label inside the search box itself, with the name of the category (or an abbreviation), that the user could remove by clicking an "x" (in which case the search would be global home?).