Use bootstrap 3 alerts

* Notice and alert classes no longer map to bootstrap the way they did
before
* Allow flashes of type info, success, danger and warning to match
bootstrap 3
* Map notice and alerts to warnings
* Use green success messages when an action succeeded, which should be
more noticeable when editing mappings
Correct GOV.UK font size

(It was slightly too small)
Use bootstrap-esque class naming

* Switch from muted-link to link-muted to match the text-muted style in
bootstrap 3 (and to avoid confusion)
Merge pull request #134 from alphagov/bootstrap-3-fixes

Bootstrap 3 fixes