Refs #294: Common bootstrap header styling

- Remove our header style rules from modules.css
- In our apps, standardize on:

    <div class="page-header">
        <h1>Page Title</h1>
    </div>

which looks okay (to me), and we can change their appearance all at once if
we want by changing one style.

Branch: feature/294-headers
Refs #294: Make main-page top padding conditional

Make the main-page top paddding conditional on being above the largest
responsive breakpoint. Once we get down to the breakpoint, it seems
unneeded and we had way too much top padding.

Branch: feature/294-headers
Merge branch 'release-0.12.0' into feature/294-headers

Branch: feature/294-headers
Merge remote-tracking branch 'remotes/origin/release-0.12.0' into feature/294-headers

Conflicts:
	rapidsms/contrib/httptester/templates/httptester/index.html

Branch: feature/294-headers
Refs #294: Fix registration side menu, headers

- Stop registration side menu text from being white-on-white by
  making the selector we use to make the responsive menu text white
  to be more selective.
- Noticed only one of the three registration pages had a header,
  fixed that while I was here.

Branch: feature/294-headers
Merge pull request #294 from rapidsms/feature/294-headers

Bootstrap header styling