Exercise comments are now editable

[JayanthBontha]

Proposed Changes

• Added feature to edit comment of exercises along with repetitions, solving issue#964
• It is a little hacky but it works and error handling is done.

Please check that the PR fulfills these requirements

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Added yourself to AUTHORS.rst

Other questions

• Do users need to run some commmands in their local instances due to this PR (e.g. database migration)? No