forums: add UX for username search results.

JIRA: FOR-627

@gwprice @marcotuts 

@marcotuts re: our chat earlier, see screengrabs below.   I made the x icon white, however the horizontal position was deliberately set up by @talbs to align well with items in the thread list, so wanted to reconfirm before messing around with that.

@lou-wang note in addition to story as scoped, threw in a trivial fix to formally tell the user that no threads matched their search, this resolved some confusion and seems generally polite, but the design isn't considered long-term.  

![screen shot 2014-06-17 at 4 00 54 pm](https://cloud.githubusercontent.com/assets/1349368/3305802/71bcedb2-f65a-11e3-90be-57f94ec43038.png)

![screen shot 2014-06-17 at 4 01 26 pm](https://cloud.githubusercontent.com/assets/1349368/3305805/7bf412ec-f65a-11e3-9b3d-486dd9848f61.png)

![screen shot 2014-06-17 at 4 01 12 pm](https://cloud.githubusercontent.com/assets/1349368/3305806/8666f578-f65a-11e3-9aab-039d7b5e0a09.png)