Download form improvements

[pconrad]

The Downloads page here needs some improvements.

Acceptance Criteria

• Free text entry of quarters in YYYYQ format is replaced with a reuse of the SingleQuarterDropdown field
• Free text entry of subject area is replaced with a reuse SubjectAreaDropdown
• Note that the backend endpoint for the second download has three additional fields that are not exposed in the form (see screenshot below). Add a level selector (using the existing react component for selecting levels.)
• Add checkboxes for the other two parameters (see Instructor search for an example of a checkbox on a React form.)

Backend endpoint:

Compare (note the missing fields):