New Git Manager options and changes

[lainets]

Description

What?

Adds new Git Manager options.

Why?

Git Manager doesn't host the exercises anymore, so we need a way to specify where the exercises are. unprotected_paths option is also required on some courses.

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.

Exercises and chapters seem to work.

Did you test the changes in

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

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

Programming style

• [X] Did you follow our style guides?

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!

[lainets]

History cleaned