[feature] use webp for thumbnails (#3116)

* update to use webp for thumbnails

* bump webp quality up to 40% from 12% (it's a bit different to jpeg quality setting)

* update to use yuva colorspace, and use thumbnail=n=10 to select frame

* fix missing comma in ffmpeg args

* add links to appropriate ffmpeg docs

* update tests

* add file size tests for thumbnails

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Co-authored-by: tobi <tobi.smethurst@protonmail.com>

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/jpeg", content.ContentType)
+	suite.Equal("image/webp", content.ContentType)
 	suite.EqualValues(testAttachment.Thumbnail.FileSize, content.ContentLength)
 }