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
Add test for finding Images by path

Functionality was added to be able to find images by their path in the database.
However, when added, it was not tested. It is now tested.

Issue: 16
Merge branch 'issues/16'