Increase button padding

* Match padding in GOV.UK elements guide
* Button padding then won’t have to be overridden here:
https://github.com/alphagov/govuk_elements/blob/master/public/sass/eleme
nts/_buttons.scss
Increase button padding to match GOV.UK

https://github.com/alphagov/static/blob/5200c818fbb2d598feff2f5af1c57ef5
fc3c2f44/app/assets/stylesheets/_buttons.scss#L15
Merge pull request #113 from gemmaleigh/increase-button-padding

Increase button padding