Fix #11969

* Add audio uploads

Fix #4827

Accept uploads of OGG, WAV, FLAC, OPUS and MP3 files, and converts
them to OGG. Media attachments get a new `audio` type. In the UI,
audio uploads are displayed identically to video uploads.

* Improve code style

Fix #11016

* Save metadata from video attachments, put correct dimensions into OG tags

* Add twitter:player for videos

* Fix code style and test

* Add spec for media controller

* Add MediaAttachment.attached scope

* Simplify methods in media controller

for local attachments, use them in URLs. Check status privacy
before redirecting to actual file.