[performance] move thumbnail generation to go code where possible (#3183)

* wrap thumbnailing code to handle generation natively where possible

* more code comments!

* add even more code comments!

* add code comments about blurhash generation

* maintain image rotation if contained in exif data

* move rotation before resizing

* ensure pix_fmt actually selected by ffprobe, check for alpha layer with gifs

* use linear instead of nearest-neighbour for resizing

* work with image "orientation" instead of "rotation". use default 75% quality for both webp and jpeg generation

* add header to new file

* use thumb extension when getting thumb mime type

* update test models and tests with new media processing

* add suggested code comments

* add note about thumbnail filter count reducing memory usage

internal/processing/media/getfile_test.go

@@ -197,7 +197,7 @@ func (suite *GetFileTestSuite) TestGetRemoteFileThumbnailUncached() {
 	suite.NoError(content.Content.Close())
 
 	suite.Equal(thumbnailBytes, b)
-	suite.Equal("image/webp", content.ContentType)
+	suite.Equal("image/jpeg", content.ContentType)
 	suite.EqualValues(testAttachment.Thumbnail.FileSize, content.ContentLength)
 }