Add admin role group using CanCanCan

[abelemlih]

• [x] Add CanCanCan gem and add a new admin role group
• [x] Add admin ability to access the flipper features dashboard and enable/disable features on the site
• [x] Add admin ability to edit front page banner
• [x] Add admin dashboard with links to features above (flipper dashboard, banner editing page)
• [x] Add link to admin dashboard to homepage

Look into the banner editing feature in Hyrax as an example.

[eporter23]

@lovinscari closing this since it is in production now.