uploads: move thumbnail generation code to MediaFile.

* Move image thumbnail generation code to MediaFile::Image.
* Move video thumbnail generation code to MediaFile::Video.
* Move ugoira->webm conversion code to MediaFile::Ugoira.

This separates thumbnail generation from the upload process so that it's
possible to generate thumbnails outside of uploads.

app/logical/media_file/video.rb

@@ -1,9 +1,29 @@
 class MediaFile::Video < MediaFile
+  extend Memoist
+
   def dimensions
     [video.width, video.height]
   end
 
-  def video
-    @video ||= FFMPEG::Movie.new(file.path)
+  def preview(max_width, max_height)
+    preview_frame.preview(max_width, max_height)
   end
+
+  def crop(max_width, max_height)
+    preview_frame.crop(max_width, max_height)
+  end
+
+  private
+
+  def video
+    FFMPEG::Movie.new(file.path)
+  end
+
+  def preview_frame
+    vp = Tempfile.new(["video-preview", ".jpg"], binmode: true)
+    video.screenshot(vp.path, seek_time: 0)
+    MediaFile.open(vp.path)
+  end
+
+  memoize :video, :preview_frame
 end