Fix for bug preventing projects be associated with channels

Reported in https://github.com/neighborly/neighborly/issues/447, the bug
appeared given recent implementation of Strong Parameters. In the
previous implementation, the channel id is added in request params.
Since Channels::ProjectsController descends from ProjectsController,
which uses the default list of parameters for any new project, not
including the channel_id in the before_filter lambda.

Also, following recent recommendations of Rails, this commit also
changes the controller filter from "<>_filter" to "<>_action".

Fixes #447.