make help center link a visible button

@stvstnfrd & @sefk - change to help modal to make the link to the help center a button. (this PR is in conjunction with the [theme one](https://github.com/Stanford-Online/stanford-theme/pull/1))

![screen shot 2014-08-14 at 4 11 03 pm](https://cloud.githubusercontent.com/assets/3364609/3928324/51c0b2ec-2408-11e4-91d9-1841bc7b37a6.png)

I have a feeling that this might look better with the HR _beteween_ the
new button and the following text. That feels like a nice, functional
divide; general help on top, specific/free-form feedback on bottom.
Thoughts?