[FEATURE] Adds a course forum

**Is your feature request related to a problem? Please describe.**
This feature allows the tutor and students to subscribe to the course to communicate on a forum by posting some messages.

**Describe the solution you'd like**
This will help the students to interact with each other and have the ability to ask the tutor about the course's content.

**Describe alternatives you've considered**
I considered the communication in the inbox but this will limit just to users.