add functional test coverage for making the file submissions disallowable on the admin interface

[redshiftzero]

Description

In #4879 we had a contribution enabling an instance to turn off file submissions - instead the instance would accept only messages. This ticket is for adding Selenium-based functional test coverage for this feature in the admin interface. The test would ideally have the following steps:

1. Admin disables file submissions.
2. Source users visits. We confirm that they cannot submit files, only messages.
3. Source submits a message.
4. Admin re-enables file submissions.
5. Source visits again and submits a file successfully.

A good starting point would be looking at similar tests here: https://github.com/freedomofpress/securedrop/blob/develop/securedrop/tests/functional/test_admin_interface.py

[DrGFreeman]

@redshiftzero, I can take this one. Getting familiar with the functional tests by now 😉.

Of course, if @wbaid wants to implement the tests (as initiator of #4879), I am fine with that too. Just advise.

[redshiftzero]

excellent, feel free to take this one @DrGFreeman 😃