Don't allow joining course outside their start/end

[mardom1]

Description of changes

Prevent joining courses that have not started yet or have already ended. This can cause issues, as the user does not have permissions to retrieve info from it.

How has this been tested?

Please describe the test strategy you followed.

• [ ] automated unit test
• [ ] automated integration test
• [ ] automated acceptance test
• [ ] manual, exploratory test

In case of manual test, please document the test well including a set of user instructions and prerequisites. Each including an action, it's result, and where appropriate a screenshot.

Checklist before requesting a review

• [ ] My code follows the coding guidelines of this project
• [ ] I have performed a self-review of my code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] My code fulfilles all acceptance criteria
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] I have made corresponding changes to the documentation
• [ ] I have added explanation of architectural decision and rationales to wiki/adr
• [ ] I have updated the changes in the ticket description

Checklist for reviewer

• [ ] The code works and does not throw errors
• [ ] The code is easy to understand and there are no confusing parts
• [ ] The code follows the coding guidelines of this project
• [ ] The code change accomplishes what it is supposed to do
• [ ] I cannot think of any use case in which the code does not behave as intended
• [ ] The added and existing tests reasonably cover the code change
• [ ] I cannot think of any test cases, input or edge cases that should be tested in addition
• [ ] Description of the change is included in the documentation