Override bootstrap pagination styles

* Move bootstrap import out of application.scss and into a separate
bootstrap and overrides file
* Add the pagination styles to this new file
* Update application to include bootstrap first, with overrides second
* Add a 4px border radius to match table styles
Rearrange border radius mixin

* Fallthrough to standards compliant unprefixed version
Improve comments and tidy up

* Override not a tweak
* .active a isn't used