Enable frozen string literals.

Make all string literals immutable by default.

app/logical/media_file/flash.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This file contains code derived from
 # https://github.com/dim/ruby-imagespec/blob/f2f3ce8bb5b1b411f8658e66a891a095261d94c0/lib/image_spec/parser/swf.rb
 #

app/logical/media_file/image.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # A MediaFile for a JPEG, PNG, or GIF file. Uses libvips for resizing images.
 #
 # @see https://github.com/libvips/ruby-vips

app/logical/media_file/ugoira.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # A MediaFile for a Pixiv ugoira file.
 #
 # A Pixiv ugoira is an animation format that consists of a zip file containing

app/logical/media_file/video.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # A MediaFile for a webm or mp4 video. Uses ffmpeg to generate preview
 # thumbnails.
 #