Unify send and receive media pipeline

While it worked so far having two different pipelines was mainly an artifact of having the pipelines modelled after two separate gst-launch commands during initial development.

This lead to some duplication of Gstreamer elements (f.e. rtcp_send_src, rtcp_send_sink, rtcp_recv_src, rtcp_recv_sink instead of a single pair rtcp_src, rtcp_sink) and is a burden when implementing SRTP due to the same sort of duplication.