Fix unit tests because of new Global object

The new Global object requires that an admin must be created before any
functionality of the site is available. So, now unit tests that don't involve
user creation start with an initialized admin user.

YEAH I TEST BRO

Issue: 12
Merge branch 'issues/12'
Adds image uploading

Images can be uploaded by POSTing to the CREATE IMAGE route. No authentication
or unit testing currently. Just barebones functionality.

Issue: 16
Show image upload form only if authorized

There is now a difference in the View if there is no user logged in. If logged
in, then the admin options are shown.

Issue: 16
Add tests for CREATE IMAGE route

Adds multiple unit tests for the CREATE IMAGE route of the Images controller.
Abstraction of the SessionsTest was necessary to promote DRY code across
SessionsTest and ImagesTest.

Issue: 16