Don't start CI when a patch series in patchwork does not touch on media files

It is a waste of CPU cycles to start a pipeline if a patch series does not touch on media files. Note that in this case 'MAINTAINERS' and '.mailmap' are not considered media files. (See issue #67 (closed)).

Also don't reply to the sender in that case.

Background: we often get dmabuf and drm patches cross-posted to linux-media, but there is no point in starting a pipeline for those.