Update manage py to use app context

[nahara7]

Status

Work in progress: Raises correct exceptions, fails test suite.

Description of Changes

Addresses #6188.

Exceptions for invalid usernames were not being raised. Instead, there was python traceback where ngettext was used.
Adds an application context before exception calls.

Testing

From the developer environment

• exec into Securedrop container
• ./manage.py add-admin # Try any 2-letter username
• ./manage.py add-journalist # Try any 2-letter username

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

[eloquence]

Hi @nahara7, just touching base - are you still interested in working further on this?

[eloquence]

Hi @nahara7, just checking in if you'd like to continue to work on this, or if you'd prefer if someone else pick this up.