Media uploads in Create Post view, refs T79

Summary:
Uses Dropzone.js to implement drag-and-drop media uploads in the Create Post
modal, including:

- link media with users in database, api
- add posts_media table to database, with fks
- add attributes to media api: array posts, int original_file_size
- add search options for media: int post, bool orphans
- automatically load and refresh orphaned files
- enable deleting orphaned files
- add oauth headers to dropzone xhr
- back dropzone files with a Media Collection

This does not implement cross referencing media with posts, it only implements
uplading files and creating media records from those files. The next diff will
include post<>model linking.

Test Plan:
- open post create modal
- upload media
- refresh page
- open post create modal, see previous upload
- delete media, see notification

Reviewers: rjmackay

Reviewed By: rjmackay

Subscribers: #ushahidi_lamu_(v3)

Maniphest Tasks: T79

Differential Revision: https://phabricator.ushahidi.com/D6