Media collection enhancements

[jimlambie]

DELETE

This PR adds the ability to send a DELETE request to a media collection, specifying a media document's _id property in the URL.

If successful, a 200 response is returned (or a 204 if feedback: false in config):

DELETE /media/5b10e5b76b600c760dc1cb93 HTTP/1.1

{
  "status": "success",
  "message": "Documents deleted successfully",
  "deleted": 1,
  "totalCount": 2
}

GET

This PR also adds support for GET requests for specific media files, e.g. http://localhost:3001/media/2018/06/02/norway.jpg

Digital Ocean Spaces

This PR also extends the S3 storage handler to support Digital Ocean Spaces. Simply add a DO access key + secret and an endpoint to configuration.

[eduardoboucas]

🚀