Upgrade to Bootstrap 3

### Upgrade Mezzanine from Bootstrap 2.3.1 to Bootstrap 3.0.

This is still work in progress, I'm opening the ticket to track progress and receive feedback. Struck out items have been completely converted to BS3.

#### Current status
- ~~Base template~~
- ~~Pages~~
- ~~Navigation menus~~
- ~~Blog~~
- ~~Twitter~~
- ~~Gallery~~
- ~~Forms~~
- ~~Pagination~~
- ~~Search results~~
- ~~Error pages~~
- ~~Ratings~~
- ~~Comments~~
- ~~User panel and accounts~~
- Inline editor
- Cartridge

#### Major changes
- Mezzanine now uses two main stylesheets that extend bootstrap.css. First, mezzanine.css adds all necessary tweaks to get BS3 working with the templates. A new file, bootstrap-theme.css, defines extra visual styles for the site, making it look more like BS2. If you want a vanilla BS3 experience, simply remove the latter file from base.html.
- Many class names from BS2 have been modified or removed from BS3. Make sure to check out the [migration notes](http://getbootstrap.com/getting-started/#migration).
- Templates now use more of Bootstrap's classes/hooks instead of implementing separate style solutions, reducing the size of mezzanine.css and necessary markup. It's recommended you become familiar with all Bootstrap components to take advantage of these resources.
- New [responsive features](http://getbootstrap.com/css/#grid-options) and a mobile-first approach are in place, giving more control on how your site displays in different screen sizes.
- The gallery now uses [Magnific Popup](http://dimsemenov.com/plugins/magnific-popup/) as image viewer. This results in less markup, responsiveness and keyboard control, among others. 

#### Known issues
- ~~Top menu displays dropdowns on click, not on hover.~~ Commit da1ded9 fixes this one, turns out we don't even need javascript! The problem right now is that mobile users have no way of displaying the dropdown in the responsive nav.

#### Other bugs found and fixed (not directly related to Bootstrap)
- There is no indication during the sign up process that the account will not be activated right away (assuming settings.ACCOUNTS_APPROVAL_REQUIRED = True). Commits d20ebe6 and [89d65b2](https://github.com/jerivas/mezzanine/blob/89d65b2fbaa9a0a447817b586bdc587569b11061/mezzanine/accounts/templates/accounts/account_signup.html#L12) add a message to indicate the situation to the user, similar to what happens already with ACCOUNTS_VERIFICATION_REQUIRED.
- When a user submitted form fails validation (be it by missing required fields or invalid input), only the fields get highlighted with errors, there's no indication at the top of the form on why the submission failed. Commit 323660d addresses this in the same way [Cartridge](https://github.com/stephenmcd/cartridge/blob/master/cartridge/shop/templates/shop/checkout.html#L55) does.
- 500 and 404 error templates used to leave the meta title blank. Fixed in 655b080.