fix: rendition urls are not FQDNs

[dopry]

Rendition URLs are returning only a path starting in 0.20.0 This updates it to return the same fully qualified domain name as the Image URLs. I would appreciate it if this could be backported to at least 0.20.0 to create 0.20.1 since I'm still on the 0.20.0 branch. I'm running off a fork in the mean time so not operationally impacted.

[dopry]

@zerolab I figured out what happened here. I didn't have WAGTAILADMIN_BASE_URL set, so my full_url wasn't generating a full url. I was still setting just BASE_URL. Core has dropped the use of BASE_URL, so maybe we should in grapple a this point. assuming full_url is working for both images and other things. Although I still think it should be using MEDIA_URL. I haven't looked into the construction of full_url. I'll look into it on tuesday.

[dopry]

Actually I take that back. I had a custom Rendition object that specified

    graphql_fields = (
        GraphQLInt("id", required=True),
        GraphQLString("url", required=True),
        GraphQLString("width", required=True),
        GraphQLString("height", required=True),
        GraphQLImage("image", required=True),
        GraphQLString("file", required=True),
    )

and seemed to be masking the grapple.types.images.ImageRenditionObject