Add AdminConstraint to routes file.

AdminConstraint ensures that the person attempting to view the route is logged
in and is an admin.
Add the Sidekiq monitoring interface to /admin/sidekiq
Link to Sidekiq monitoring in new Admin dropdown
Merge branch 'admin-authentication'

This adds an AdminConstraint to routes.rb, denying access to any non-admin
user.

The Sidekiq monitoring app is also added, mounted at /admin/sidekiq, and
this is exposed through a dropdown accessible in the header to admins.
Update gems
Add a render_json_partial method to maintain consistency

render_json_partial(name, locals, extras)

The given partial *name* is rendered with the given locals and added to
a hash as the content key. Any extras are merged in and the result is
rendered as JSON.