[feature] support processing of (many) more media types (#3090)

* 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

internal/federation/dereferencing/emoji_test.go

@@ -75,7 +75,7 @@ func (suite *EmojiTestSuite) TestDereferenceEmojiBlocking() {
 	suite.Equal("image/gif", emoji.ImageContentType)
 	suite.Equal("image/png", emoji.ImageStaticContentType)
 	suite.Equal(37796, emoji.ImageFileSize)
-	suite.Equal(7951, emoji.ImageStaticFileSize)
+	suite.Equal(9824, emoji.ImageStaticFileSize)
 	suite.WithinDuration(time.Now(), emoji.UpdatedAt, 10*time.Second)
 	suite.False(*emoji.Disabled)
 	suite.Equal(emojiURI, emoji.URI)