Add /admin route and page that only Admins can access.

Add a special 'admin' interface that would allow authorized people to do maintenance on the app. For example deactivate/delete spam users and/or projects, load the static pages, 'pin' important projects to the top of the Projects page.

Options:
- rolify and pundit gems
- https://github.com/activeadmin/activeadmin
- https://docs.avohq.io/2.0/installation.html (newer)

Feature: Admin page
As an admin user
So that I can perform admin tasks
I want to be able to use the admin page

Scenario: Access admin page
Given I have logged in as an admin
When I am on the “Administration” page
Then I should see “AV Administration” on the page

Scenario: Deny access to admin page
Given I have logged in as a non-admin
When I try to go to the “Administration” page
Then I should see an error message