Add locations index view & logic for super admin vs. regular admin.

- Redirect admins to locations index view after signing in
- Admin must be signed in to access locations index view
- Add 'Your locations' link to nav bar
- Add super_admin boolean column to Admins table and set to false by default
- Update INSTALL.md with instructions for setting an admin as a super admin
- Create admins to test with in db/seeds.rb