help modal button style

@stvstnfrd & @sefk - change to help modal to make the link to the help center a button. (this PR is in conjunction with the [platform one](https://github.com/Stanford-Online/edx-platform/pull/108))

![screen shot 2014-08-14 at 4 11 03 pm](https://cloud.githubusercontent.com/assets/3364609/3928327/5af7f19a-2408-11e4-8d69-7d5b316425f9.png)

In one of @sjang92 's recent PRs, I remember seeing him use a
`.button-primary` or similar class name which added the extra emphasis
to the button (blue vs gray background, in this case); is this something
that could be adapted here instead of explicit styling?