Attachments and user photo storage refactor.

Deploy steps:

- Apply all migrations.
- Configure nginx to serve media files on https://media.taiga.io/
- run `python manage.py migrate_attachments` (not trully transactional because it copies files to new location that can not be done in a transaction)

NOTES:
- The https://media.taiga.io url is hardcoded in a migration script because it should be the final domain for serve files.
- MEDIA_URL should be set to "https://media.taiga.io/" on local.py for proper attachments url generation.