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)

Could you split the anchor tag out onto its own line, mirroring the
buttons below? Since its visually separate from the preceding text, it's
easy to grok the markup if it's on its own line.