Functional tests fail to add admins if requested

[heartsucker]

Description

Reporting this here while documenting TODOs (#724).

From securedrop/tests/functional/journalist_navigation_steps.py:

        if is_admin:
            # TODO implement (checkbox is unchecked by default)
            pass

Steps to Reproduce

Attempt to add an admin during functional tests.

Expected Behavior

An admin is added.

Actual Behavior

A regular use is added.

[eloquence]

The current code is:

        if is_admin:
            raise NotImplementedError("Admin's can't be added yet.")

Problematic apostrophe usage aside, since it's now at least raising an error I'm removing the bug label.

[deepthinidwannaya]

I'm working on this. I've implemented the logic to select the checkbox and added a testcase for an admin creating an admin user and logging in with the newly created admin.

[heartsucker]

Closed by #3854