[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/api/client/instance/instancepatch.go

@@ -182,13 +182,6 @@ func validateInstanceUpdate(form *apimodel.InstanceSettingsUpdateRequest) error
 		return errors.New("empty form submitted")
 	}
 
-	if form.Avatar != nil {
-		maxImageSize := config.GetMediaImageMaxSize()
-		if size := form.Avatar.Size; size > int64(maxImageSize) {
-			return fmt.Errorf("file size limit exceeded: limit is %d bytes but desired instance avatar was %d bytes", maxImageSize, size)
-		}
-	}
-
 	if form.AvatarDescription != nil {
 		maxDescriptionChars := config.GetMediaDescriptionMaxChars()
 		if length := len([]rune(*form.AvatarDescription)); length > maxDescriptionChars {