Resolve "Access Denied exception on returning after enroll"

What does this mr do?

After a student enrolled for a course edition, going back leads to an Access Denied error. This should not be the case, instead it should be that the student is informed that they are already enrolled for the edition or they are redirected to the catalog of courses.

Actions taken to fix bug

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made and the bug I fixed.
• A test was created to test the bug.
• I have updated the documentation accordingly.
• I adhere to the style guide.

closes #107 (closed)