Add Admin interface to API app

This PR adds an admin interface to the API app. It behaves mostly like the standalone one, but with many improvements. The admin interface can be accessed by visiting the `/admin` path and signing in with one of the three admin users defined in `db/seeds.rb`. To set an admin as a super admin, see the last section in INSTALL.md for instructions.

When a regular admin signs in, they will see 2 buttons that they can click to see the Organizations and Locations they have access to. Super admins will have 2 additional buttons to create an Organization or a Location. Only super admins can create a new Organization. Regular admins can create a new location for the Organizations they have access to. The "Add a new location" button will appear in the `/admin/locations` path for regular admins.

Admins can now view, edit, and create multiple services for each location. To view the services that exist for a location, go to the location, then scroll down to the Services section at the bottom. From there, an admin can view or create a Service.

Note that the `name` and `description` attributes of a Service are now required.

I believe that covers all the new improvements. I would love to get feedback on the UX. Try it out and let me know what you think.