Implement role filter on forum posts

[jullia02]

An additional 'role' button is added, where users can choose to filter posts by either students or instructors.

Estimated effort: 6+ hours
Dependencies:
Acceptance criteria:

• When no role is selected, all posts should be displayed.
• When the 'instructor' role is selected only posts by instructors should be displayed.
• When the 'students' role is selected only posts by students should be displayed.
• Double clicking on a role should undo the filter.