Setup basic page support

- Broken up index.html into header.html, nav.html, and footer.html to
  allow reusability on other django pages
- Use sites/flatpages django app for static pages (about/documentation)
  - These are exported to 'sana_builder/fixtures' folder and have to be
    imported/exported with `sana_builder/manage.py`
- Added barebone structure for Ember on `/editor` page

JIRA: SPB-50
Merge pull request #31 from SanaMobile/50-setup-basic-pages

Setup basic page support