Standardise radio buttons and checkboxes

• update gemifile
• add necessary js references and code
• add necessary class to the form labels

CSS styles for this can be found in static here -
https://github.com/alphagov/static/pull/449

https://www.agileplannerapp.com/boards/105200/cards/5197
If the `.selectable` CSS is on static, would it be better to include the JS there too? We could include `selection-button.js` into the static template, and instantiate it there. Then each app only needs to apply the `.selectable` class to get styles and behaviour.

(This is much closer to the model Components is using, and means less work when adding things to new apps)