Bug 748503: Port Firefox nightly firstrun page to Bedrock

- Created new page with content copied from existing page
- Mark for translation
- Took a stab at formatting, but needs more work
- Use /firefox/nightly/firstrun/ instead of
  /projects/firefox/prerelease.html

- Added rewrite rules so old page:
   /projects/firefox/prerelease.html
ends up at
   /firefox/nightly/firstrun/
served with new content from Bedrock.

Add link to nightly build bug team

Link requested in https://bugzilla.mozilla.org/show_bug.cgi?id=864541
Was added on the php/svn side, needed to be added here too so the
ported page would have it when it goes live.

Page styling
Add CONTRIBUTING file for github notification coolness.
Merge pull request #855 from pmclanahan/add-contributing-file

Add CONTRIBUTING file for github notification coolness.