Migrate to Bootstrap 3

This PR moves the site to bootstrap 3. If there are performance concerns, we can alleviate these by using a CDN.

The primary reason for moving is two-fold:

1. Super easy to be really responsive; the previous iteration had lots of minor issues on mobile (most pages had horizontal scrolling)
2. Contributing is much easier with a known framework

Note: **This includes the new ratings changes that I made in #112

More info:

- Now actually responsive!
- Adds sidebar
  - Login, takes you back to the same page
  - Splits to have event info on top of page, login below page
  - Combell logo disappears on mobile, as it's directly above the footer which also contains it
- Added a link to github in the footer (left)
- Nicer pager
- Clearer (actionable) "I will be attending"/"I attended" text+buttons
- Short URLs now have a "Copy to Clipboard" UI
  - uses flash but:
  - is hidden on mobile/tablet
  - will highlight the text in the input when focused, or clicked without flash
- Comments have a new interface
  - Supports gravatar
  - Comment shown above author/date/via
  - via is hidden on mobile
  - When ratings are shown they are also responsive:
    - Horizontal/on-top for desktop
    - Vertical/left for small/tablet
    - Vertical/on-top for mobile
- Twig will check for local images before linking to the old joind.in site