Multi-value filters should run validate() on each value

[jeremystretch]

Proposed Changes

We use multivalue_field_factory() to generate filters that accept multiple values, to support querying for e.g. ?status=a&status=b (applying an OR logic). However, these filters are not properly validating the passed values. The class returned by multivalue_field_factory() should have its run_validators() and validate() methods overriden so that they execute on each value in the list.

Justification

This problem was originally uncovered by @arthanson while working on #10348. I've broken it into a separate issue for explicit tracking of the change, as it affects all multi-value filters.