Allow to filter interface translation for "context" with "contains" (LIKE)

Description of the need

Backdrop ships with config translation. However, it's currently not possible to filter by translation context on admin/config/regional/translate/translate.

It would be much easier to find config translation items with search strings like, for example, config:system*

Proposed solution

Add the context as filter to the query.

Alternatives that have been considered

Improve views handlers regarding search (also lacks support for context filtering) and turn that page into a view - might be a little more work, though.

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes a filter for translation context on User interface translation (admin/config/regional/translate/translate).