Rearrange CSS assets for access to bootstrap SASS

* Instead of automatically including all, switch to being specific
about what’s being included and in what order
* This gives all subsequent CSS imports access to bootstrap SASS
variables and mixins while opening the classes up to extension.
Remove fix for bootstrap 3.0.3 bug

* Warning class on striped table rows has been fixed in bootstrap 3.1
Remove unnecessary error styles

* Old error styles seem to have crept back in after an earlier refactor
* The error explanation styles are superseded by bootstrap alert and
alert-danger classes
Use computed line height for vertical margins

* Bootstrap has a variable that does this calculation already, with an
additional floor() to round pixel values.
* See http://getbootstrap.com/customize/#typography
Rename GillSans variable to match bootstrap style
Update style guide to point to new bootstrap gem

* Include a link to mixins, they’re otherwise hard to find (ie not on
bootstrap docs pages)
Merge pull request #176 from alphagov/a-better-css-pipeline

Update asset pipeline for access to bootstrap SASS