Support restricting queries by type (see #317)

Uses `filter` as the query parameter. The obvious choice of `type`
is currently not possible due to a bug in the Play framework:
https://github.com/playframework/playframework/issues/2259

However, `filter` allows for a generalized approach that is not
restricted to the actual type (we could filter by language, etc),
which aligns pretty well with what we do with `id` or `subject`
(which both map to multiple fields, providing a data abstraction).