Add support for different media buckets

[eduardoboucas]

At the moment, all images are being uploaded to the same media bucket in API (whatever is set as the default one). Calls to API wrapper should be configured to use whatever is defined in settings.collection on the Reference field as the name of the bucket, so we can have different buckets for article images, user avatars, etc.

(Not a difficult change, just documenting to make sure I don't forget to do it.)

[eduardoboucas]

At the moment, Publish identifies a reference to a media collection by looking at a hardcoded mediaStore value in the settings.collection value from the field schema. To implement this ticket, we need to make this lookup process dynamic.

To make that happen, API needs to inform what the various media buckets are so we can know whether a collection is a media bucket or not. I've suggested an approach for that.

I'll revisit this ticket once that functionality has been agreed.

[danwdart]

This will require the API component first I think, then Publish can piggy back off of my and Eduardo's existing work.

[eduardoboucas]

Needs to get back on the roadmap, as per conversation with @josephdenne. Added status: needs UI label, as we need to understand how different media buckets would be displayed on the interface.