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

db/migrate/20190420025523_add_blurhash_to_media_attachments.rb

@@ -0,0 +1,5 @@
+class AddBlurhashToMediaAttachments < ActiveRecord::Migration[5.2]
+  def change
+    add_column :media_attachments, :blurhash, :string
+  end
+end