🐛(search) combine several values of a choices filter with OR

[Tiago-Salles]

Purpose

When several values of a choices search filter are selected, the search returns no results. For example, on the demo site, selecting the weekly paces "One to two hours" and "More than two hours" returns nothing, while each of them alone returns courses:

Query	Results
?pace=1h-2h	1
?pace=gt-2h	1
?pace=1h-2h&pace=gt-2h	0

The cause is in ChoicesQueryMixin.get_query_fragment: it returns one query fragment per selected value, and CourseSearchForm.build_es_query flattens every fragment into a single bool must clause. Each selected value therefore becomes an extra AND condition, and since a course run has a single pace, no document can match two of them.

Terms based filters (subjects, levels, organizations, languages) are not affected because TermsQueryMixin emits a single terms query, which is an OR by nature. Only the filters built on ChoicesQueryMixin (pace, availability) have the problem.

Proposal

When more than one value of a choices filter is selected, wrap the fragments of the selected values in a single bool should clause with minimum_should_match: 1, so the values are ORed together while the filter as a whole is still ANDed with the other filters. Each value's fragment is a list of clauses that must all match (availability values have two range clauses), hence the inner bool must.

• Combine the fragments of several selected values with should in ChoicesQueryMixin.get_query_fragment; a single selected value keeps the exact previous output and the fragment key is unchanged, so the facet aggregations that exclude a filter's own fragments keep working
• Add unit tests for StaticChoicesFilterDefinition.get_query_fragment with one and several values
• Add an Elasticsearch test selecting two paces and expecting the union of both course sets
• Add a changelog entry

Related to: #2532

[Tiago-Salles]

Hey, @jonathanreveille and @liamls, could you please review this? Thanks!