Add media dimensions (#2448)

* Fixes #1985

- add migration AddMediaAttachmentMeta, which add meta field to media_attachments
- before saving attachment, set file meta if needed
- add meta in api

* add spec

* align the “size” format for image and video

* fix code climate

* fixes media_attachment_spec.rb

db/migrate/20170425131920_add_media_attachment_meta.rb

@@ -0,0 +1,5 @@
+class AddMediaAttachmentMeta < ActiveRecord::Migration[5.0]
+  def change
+    add_column :media_attachments, :file_meta, :json
+  end
+end