Add the ability to filter requests by course subject

[rmellema]

Note: for support questions, please mail tutoring.svcover.nl. This repository's
issues are reserved for feature requests and bug reports.

• I'm submitting a ...
  • bug report
  • feature request
  • support request => Please do not submit support request here, see note
    at the top of this template.

(Select the correct option by putting a x in the box)

• What is the current behavior?
  Currently there is no filtering for requests

• What is the expected behavior?
  The ability to filter requests by subject

• What is the motivation / use case for changing the behavior?
  This makes it easier for people to find people they can give tutoring to

• Please tell us about your environment:

  • Browser: all

• Other information (e.g. detailed explanation, stacktraces, related issues,
  suggestions how to fix, links for us to have context, eg. stackoverflow,
  gitter, etc)