Add blurhash (#10630)

* Add blurhash

* Use fallback color for spoiler when blurhash missing

* Federate the blurhash and accept it as long as it's at most 5x5

* Display unknown media attachments as blurhash placeholders

* Improve style of embed actions and spoiler button

* Change blurhash resolution from 3x3 to 4x4

* Improve dependency definitions

* Fix code style issues

lib/paperclip/blurhash_transcoder.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module Paperclip
+  class BlurhashTranscoder < Paperclip::Processor
+    def make
+      return @file unless options[:style] == :small
+
+      pixels   = convert(':source RGB:-', source: File.expand_path(@file.path)).unpack('C*')
+      geometry = options.fetch(:file_geometry_parser).from_file(@file)
+
+      attachment.instance.blurhash = Blurhash.encode(geometry.width, geometry.height, pixels, options[:blurhash] || {})
+
+      @file
+    end
+  end
+end