Feature/esckan-35 - Composer REST new generic endpoint - Knowledge statement

A new generic endpoint for public Knowledge statement.

Add new endpoint with new serializer - KnowledgeStatementSerializer and filterset - KnowledgeStatementFilterSet.

Only GET is allowed - is tagged "public".

filter by 4 query parameters - origin, via, destination, population uris Only exported Connectivity statements are shown. As mentioned in lucidcharts

Default behaviors of the filters. Example:

When multiple population uris are selected - OR is applied.
When one population uri and one via/origin/destination is selected - AND is applied

MetaCell / sckan-composer