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

app/serializers/rest/media_attachment_serializer.rb

@@ -5,7 +5,7 @@ class REST::MediaAttachmentSerializer < ActiveModel::Serializer
 
   attributes :id, :type, :url, :preview_url,
              :remote_url, :text_url, :meta,
-             :description
+             :description, :blurhash
 
   def id
     object.id.to_s