Now that the Jabber settings functionality is refactored into its
own Django app, we don't need this test in the Courseware Django
app.

Rename get_password_for_user to get_or_create_...

The new name describes more accurately what the function does: it
creates a new password for the user if it can't find one in the
database.

Move DEFAULT_PASSWORD_LENGTH to JabberUser model

This seems more appropriate as a property of the model as opposed
to a loose constant in the utils module.

Random string generation returns proper length

Before, the length was only accurate if it was a multiple of four.
Now, we calculate the number of bytes needed from /dev/urandom
properly and we truncate the string to just the right length.
(Base64 returns strings that are a multiple of four in length, but
we truncate since we won't ever care to decode the string.)

Adding working utility tests for Jabber app

The majority of the functionality of the Jabber app is in the utils
module, so we test those functions thoroughly.

The test settings don't currently have settings to connect to a
Jabber database (need @jrbl's help to get the migration files to set
it up properly), so we temporarily skip tests that hit the Jabber
users table.

Add migration for Jabber users table for testing

In testing environments, we want to be able to interact with the
database as we create JabberUser instances. This commit adds a
migration for the purposes of testing; the migration should *not*
be used for anything else though. The actual Jabber users table
should be created by ejabberd during provisioning.

Unskip Jabber tests that interact with database

Now that we have a migration for the Jabber users table in the test
environment, we can hook it up in test and "unskip" the tests that
hit that table.