[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

testrig/testmodels.go

@@ -1028,7 +1028,7 @@ func NewTestAttachments() map[string]*gtsmodel.MediaAttachment {
 			Thumbnail: gtsmodel.Thumbnail{
 				Path:        "01F8MH5ZK5VRH73AKHQM6Y9VNX/attachment/small/01FVW7RXPQ8YJHTEXYPE7Q8ZY0.jpg",
 				ContentType: "image/jpeg",
-				FileSize:    19312,
+				FileSize:    11751,
 				URL:         "http://localhost:8080/fileserver/01F8MH5ZK5VRH73AKHQM6Y9VNX/attachment/small/01FVW7RXPQ8YJHTEXYPE7Q8ZY0.jpg",
 				RemoteURL:   "http://fossbros-anonymous.io/attachments/small/a499f55b-2d1e-4acd-98d2-1ac2ba6d79b9.jpg",
 			},
@@ -1205,7 +1205,7 @@ func NewTestEmojis() map[string]*gtsmodel.Emoji {
 			ImageContentType:       "image/png",
 			ImageStaticContentType: "image/png",
 			ImageFileSize:          36702,
-			ImageStaticFileSize:    10413,
+			ImageStaticFileSize:    6092,
 			Disabled:               util.Ptr(false),
 			URI:                    "http://localhost:8080/emoji/01F8MH9H8E4VG3KDYJR9EGPXCQ",
 			VisibleInPicker:        util.Ptr(true),
@@ -1227,7 +1227,7 @@ func NewTestEmojis() map[string]*gtsmodel.Emoji {
 			ImageContentType:       "image/png",
 			ImageStaticContentType: "image/png",
 			ImageFileSize:          10889,
-			ImageStaticFileSize:    10808,
+			ImageStaticFileSize:    8965,
 			Disabled:               util.Ptr(false),
 			URI:                    "http://fossbros-anonymous.io/emoji/01GD5KP5CQEE1R3X43Y1EHS2CW",
 			VisibleInPicker:        util.Ptr(false),