Remove useless test_submit_clean_file test

This test is leftover from when we had MAT integrated to attempt to
automatically remove file metadata. It doesn't add any additional
coverage beyond what test_submit_file covers, so I'm removing it to get
rid of clutter.
Rename test_setup.py to common.py

The convention (followed by py.test and others) is to expect files with
a test_* prefix to contain tests. test_setup.py contains common setup
and teardown functions for SecureDrop, so I renamed it something less
confusing.
Update tests to work with journalist login flow

Fixes tests that were broken by the new login flow. Also adds a
functional test for logging in and some (not enough) unit tests for
aspects of the admin interface.