Allow search API results to be sorted by department course numbers

[mbertrand]

User Story

When searching for resources, the user should be able to sort results by course number if desired. When a department is selected, the sort should apply only to courses within that department.

Description/Context

sort_coursenum was added to indexed course documents in #173. The search API should allow sorting on that field if specified in query parameters.

This is a port of existing functionality from open-discussions.

• Issue: Make OCW courses searchable and sortable by extra course numbers open-discussions#3453
• PR: sort by department coursenum when there is a department filter open-discussions#3457 and department course number sort ocw-hugo-themes#134

Acceptance Criteria

• Search results for courses ordered by sort_coursenum if query parameter is passed.
• Search results for courses ordered by sort_coursenum only within selected department(s) if any.

Plan/Design

• Add an optional query parameter that the search API accepts to sort by sort_coursenum, ascending or descending.
• Make sure the option is documented in the API schema

[Ferdi]

What are current sort options for /search api ?

[mbertrand]

id, readable_id, last_modified, runs.start_date (ascending and desscending).

[mbertrand]

Old open-discussions issue, the functionality should work about the same here: mitodl/open-discussions#3453

[Ferdi]

A good name for this field would be mitcoursenumber or mitcoursenr

Unless there a reason the field to start with "sort_"?

[mbertrand]

A good name for this field would be mitcoursenumber or mitcoursenr

Unless there a reason the field to start with "sort_"?

That's doable, I can adjust it.

[mbertrand]

@Ferdi you okay with making that mit_course_number or just course_number? I feel it might be a bit more readable to add some underscores in there.