Reset flag reason form after submit

Applies to https://github.com/GigaOM/gigaom/issues/5280

Before screens in ticket.
*Test process* 
1. click flag icon 
1. select Other, enter text
1. click "Flag" button
1. Give it a sec and un-flag. 
1. Click "Flag" again should see:
![screen shot 2014-09-11 at 10 01 51 am](https://cloud.githubusercontent.com/assets/929375/4235014/38c2cbe4-39bc-11e4-9a6b-e851ef22ed04.png)
repeat with another option ("Spam"), result should be the same.