Merge pull request #337 from fsteeg/issue-315-primary

Always explicitly identify the primary topic
Support restricting queries by type (see #317)

Uses `t` as the query parameter. The obvious choice of `type` is
currently not possible due to a bug in the Play framework [1], so
we will set up a rule in our Apache proxy to rewrite `type` to `t`
until the issue is resolved. We can then switch from `t` to `type`
internally and remove the rewrite, without affecting API clients.

[1] https://github.com/playframework/playframework/issues/2259
Remove RDF type from properties to use for detail resolution (#317)

The type of the main entity is always included, since that triple
has the main entity as subject. The resolved entities are all
included for their labels. Their types cause issues when
restricting queries by type. Since a solution that retains these
types would be quite involved, and we have no requirement for them
to be included, we simply remove them from the resolution config.