FEATURE: ?include_raw parameter for /t/id/posts.json

include_raw is not added for the wordpress view because it uses the
BasicPostSerializer, and is not a one-line change.

This is the only use of the TopicViewPostsSerializer class, and the
previous change covered the only use of the TopicViewSerializer class.
No other locations include the PostStreamSerializerMixin. Therefore,
this feature is most likely complete.
Merge pull request #2955 from riking/include_raw_2

FEATURE: ?include_raw parameter for /t/id/posts.json