Allow teachers to add any normal links to the course menu

[ihalaij1]

Description

What?

Allow teachers to add any normal links to the course menu

Why?

Normal links do not transmit personal data (like LTI launch does), so we need a balance between user experience and overzealous security and data privacy.

Fixes #563

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.

Tested that adding links is now possible by teachers.

Did you test the changes in

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

Translation

• [x] Did you modify or add new strings in the user interface?

Programming style

• [x] Did you follow our style guides?
• [ ] 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.
• [ ] 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