Blaze it

* Fix #117 - Add ability to specify alternative text for media attachments

- POST /api/v1/media accepts `description` straight away
- PUT /api/v1/media/:id to update `description` (only for unattached ones)
- Serialized as `name` of Document object in ActivityPub
- Uploads form adjusted for better performance and description input

* Add tests

* Change undo button blend mode to difference

* 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

* Fix #2108 - Fix gif uploads
Add specs for media attachment gifv conversion

* Add ffmpeg to travis

* Make travis install ffmpeg, not libav

* Switch travis to trusty

as well as formatting of content