Implement git manager link

[mikaelGusse]

Description

What?

Add a link to the teacher view's gitmanager tab which allows the user to easily go to the gitmanager page and automatically log in.

Why?

Makes it easier to debug issues regarding courses and streamlines the process of logging in to the gitmanager site

How?

Added a link to the page that user javascript to fetch the correct url to the gitmanager instance and also gets the auth token through the A+ api which is passed using a HTTP POST request.

Fixes #1027

Testing

Remember to add or update unit tests for new features and changes.

• How to test your changes in A-plus
• How to test accessibility

What type of test did you run?

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

Added the change into test-plus and checked that it truly logs the user into gitmanager.

Did you test the changes in

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

Think of what is affected by these changes and could become broken

Translation

• [x] 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.
• [ ] 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!