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

app/views/api/v1/statuses/_media.rabl

@@ -3,3 +3,4 @@ attributes :id, :remote_url, :type
 node(:url)         { |media| full_asset_url(media.file.url(:original)) }
 node(:preview_url) { |media| full_asset_url(media.file.url(:small)) }
 node(:text_url)    { |media| media.local? ? medium_url(media) : nil }
+node(:meta)       { |media| media.file.meta }