* initial work replacing our media decoding / encoding pipeline with ffprobe + ffmpeg
* specify the video codec to use when generating static image from emoji
* update go-storage library (fixes incompatibility after updating go-iotools)
* maintain image aspect ratio when generating a thumbnail for it
* update readme to show go-ffmpreg
* fix a bunch of media tests, move filesize checking to callers of media manager for more flexibility
* remove extra debug from error message
* fix up incorrect function signatures
* update PutFile to just use regular file copy, as changes are file is on separate partition
* fix remaining tests, remove some unneeded tests now we're working with ffmpeg/ffprobe
* update more tests, add more code comments
* add utilities to generate processed emoji / media outputs
* fix remaining tests
* add test for opus media file, add license header to utility cmds
* limit the number of concurrently available ffmpeg / ffprobe instances
* reduce number of instances
* further reduce number of instances
* fix envparsing test with configuration variables
* update docs and configuration with new media-{local,remote}-max-size variables
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