[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/admin/emojiget_test.go

@@ -60,7 +60,7 @@ func (suite *EmojiGetTestSuite) TestEmojiGet1() {
   "id": "01F8MH9H8E4VG3KDYJR9EGPXCQ",
   "disabled": false,
   "updated_at": "2021-09-20T10:40:37.000Z",
-  "total_file_size": 47115,
+  "total_file_size": 42794,
   "content_type": "image/png",
   "uri": "http://localhost:8080/emoji/01F8MH9H8E4VG3KDYJR9EGPXCQ"
 }`, dst.String())
@@ -92,7 +92,7 @@ func (suite *EmojiGetTestSuite) TestEmojiGet2() {
   "disabled": false,
   "domain": "fossbros-anonymous.io",
   "updated_at": "2020-03-18T12:12:00.000Z",
-  "total_file_size": 21697,
+  "total_file_size": 19854,
   "content_type": "image/png",
   "uri": "http://fossbros-anonymous.io/emoji/01GD5KP5CQEE1R3X43Y1EHS2CW"
 }`, dst.String())