Add to context if Use Cases are filtered on

[sodevious]

http://localhost:8000/case-studies/

I was able to easily update the context if a region was filtered on. I'd like to do the same if a use case is filtered on.

See: https://github.com/rapidpro/chpro-microsite/commit/1cad18f941d6cefdaaf742c2f47a2bee5a8189b2