[chore]validates comment's content not be nil
[chore]validate content on frontend
[chore]make tests pass
[chore]fix fancybox not show up first time load bug

Note: based on [this
SO](http://stackoverflow.com/questions/18105848/jquery-fancybox-doesnt-show-only-after-page-reload)
Merge pull request #302 from peterzd/blank_comment

Blank comment