Change transcode web interface (#377)

Target format is now a subresource of /trans. There's also a named
parameter "bitrate" to pass along to transcoders.
Add unit test for httphandler.trans()
transcode: Make bitrate < 1 alias for default value
Adapt frontend to new transcode api call
Merge branch 'devel' into transcode
Add user option to force (transcoding to) bitrate
Improve force bitrate option UX
Don't overwrite error flashes so none will be missed
Bitrate option UI depends on media.transcode config
Enable bitrate limiting in playlist mgr (#377)
Fix bitrate user option displays
Merge branch 'transcode' into devel