Restricting the option to "Mark a response as the best response" To instructors only

[Procos12]

#16 Allowed all users to mark a response as the best response. Finishing up the requirement in the second user story, we should limit this feature only to instructors.

Estimated Effort:
Around 1 week and a half to two weeks. Not sure if limiting the option to instructors only would be medium-effort or a lot of effort.

Dependencies:
#16

Initial assignments for team members:
Abdulwahab Al-Rumaihi

Milestones to indicate which sprint the issue should be completed in:
Should be completed by sprint 2

Acceptance criteria:
The functionality will be tested on a live version of nodeBB by the team. To accept this, we will experiment and see if all instructors have access to this feature and that no students are able to mark a response as the best response.