Fix Task Def model and service to keep track of Numbas config

[satikaj]

Description

Fixed the bug where although the Numbas test was uploaded, the delete and download buttons did not show in the editor on refresh. The other config options are also now stored in the backend (corresponding doubtfire-api PR: thoth-tech/doubtfire-api#16).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Opening the task editor as a convenor, you can upload and configure Numbas tests.

Before:

Although a test was uploaded and configured, the form would be blank on refresh:

After:

Testing Checklist:

• Tested in latest Chrome
• Tested in latest Safari
• Tested in latest Firefox

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[ublefo]

Tested and it's all working well, two things that we could improve:

1. Hide all options in the numbas component if it's disabled to reduce visual clutter
2. Disable numbas test automatically when saving the task definition if the test data is not uploaded or has been deleted.