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.