Update admin gem to 1.0.0
Replace silent margin classes with mixins

* Mixins aren’t using !important
* Generated CSS is cleaner
* Silent classes have been removed from admin gem
Explicitly pad list group items on site dashboard

* Before the default Bootstrap styles were being modified
* Instead use new admin gem helper class
Force link styles in filters to inherit colour

* Increased specificity of visited link styles in admin gem caused
filter options to go blue, prevent this from happening again
Merge pull request #317 from alphagov/upgrade-admin-gem

Upgrade admin gem to 1.0.0