Upgrade Bootstrap from 2.0.3 to 2.3.2

* Delete old Bootstrap js and css assets
* Use Bootstrap SASS gem to load assets and to give access to SASS
variables and mixins
* Bootstrap changelog: https://github.com/twbs/bootstrap/releases
Switch back to smaller font size and line height

* Doesn’t address oversized headers
Simplify govspeak help styles

* Remove font-size declaration, was compiling to invalid CSS
* The background is already white
* The width doesn’t need to be constrained
* The pre styles aren’t being used, and it’s grey by default
* Remove the link cursor styles and add an href to the links themselves
(Bootstrap 2.3.2 defaults a target to the value of a link’s href)
Reduce govspeak header sizes

* Bootstrap 2.3.2 has larger headings, scale them back for govspeak so
the aesthetics stay the same
Use list inline styles provided by Bootstrap 2.3.2

* 2.0.3 missed the `inline` feature so it was patched in, this patch
can be removed now
Update `form-large` styles to match new buttons

* Large buttons are a bit bigger, this more specific version needs to
match that
Update Whitehall style guide advice

* Link to guidance about 2.3.2, especially the fluid grid
Remove margin from h3s in admin document lists

* Bootstrap 2.3.2 adds a default margin to h3s
* In this case it’s undesirable, styles now match 2.0.3
Merge pull request #1669 from alphagov/bootstrap-upgrade

Upgrade Bootstrap to 2.3.2