Research Classes page: filter all the options by project_id

[pjanik]

https://www.pivotaltracker.com/story/show/187422622

This is necessary, when classes are shared by multiple teachers. For example:

• Class 1 has Teacher A and Teacher B.
• Teacher A is a member of Project 1: Cohort 1.
• Teacher B is a member of Project 2: Cohort 2.
  Class 1 is considered valid for both Project 1 and Project 2. Since we use the class output table as a source for all filter options, even if we're looking at "Project 1" page, "Project 2: Cohort 2" and "Teacher B" will appear in the filter options.

This can be fixed by explicitly filtering all the options by project_id.

I've updated the specs to test this case (by adding a second teacher to class1).

[dougmartin]

Looks good 👍