Add support to configure module model solution

[Mikael-Lenander]

Description

What?

Adds support to configure new course module options: model answer and module solution reveal rule. See https://github.com/apluslms/a-plus/pull/1229. Should be merged together with https://github.com/apluslms/gitmanager/pull/51 and https://github.com/apluslms/a-plus/pull/1250.

Why?

Options should be able to be configured in the courses' repositories.

How?

Meta directive accepts new options.

Testing

What type of test did you run?

• [ ] Accessibility test using the WAVE extension.
• [ ] Django unit tests.
• [ ] Selenium tests.
• [ ] Other test. (Add a description below)
• [x] Manual testing.

Added rst code to O1 and Aplus manual repositories that configure new course module options. Ran gitmanager and A+ locally and checked in A+ course settings that the options were saved.

Did you test the changes in

• [ ] Chrome
• [x] Firefox
• [ ] This pull request cannot be tested in the browser.

Translation

• [ ] Did you modify or add new strings in the user interface? (Read about how to create translation)

Programming style

• [x] Did you follow our style guides?
• [x] Did you use Python type hinting in all functions that you added or edited? (type hints for function parameters and return values)

Have you updated the README or other relevant documentation?

• [ ] documents inside the doc directory.
• [x] README.md.
• [ ] Aplus Manual.
• [ ] Other documentation (mention below which documentation).

Is it Done?

• [ ] Reviewer has finished the code review
• [ ] After the review, the developer has made changes accordingly
• [ ] Customer/Teacher has accepted the implementation of the feature

Clean up your git commit history before submitting the pull request!