Remove hardcoded "pdf" path from IIIF asset base url

[ctgraham]

Is your feature request related to a problem? Please describe.

We would like to allow digital objects to be rendered from an arbitrary IIIF endpoint, but the code currently hardcodes the IIIF endpoint in the form of:
settings.ASSET_BASEURL + "/pdfs/" + identifier

pisces/transformer/mappings.py

Line 41 in cfe6a91

req = requests.head("{}/pdfs/{}".format(settings.ASSET_BASEURL.rstrip("/"), identifier))

Describe the solution you'd like

Move the "/pdfs/" reference to the ASSET_BASEURL setting.

Describe alternatives you've considered

Since we are checking on assets which we expect to render in an IIIF viewer, perhaps the check should ultimately be for the IIIF manifest rather than a HTTP HEAD of the IIIF viewer?

Or perhaps it would be better to split this into two settings: ASSET_BASEURL and ASSET_BASEPATH, considering "Additional context" for DIMES?

Additional context

This may have implications for the setting of REACT_APP_S3_BASEURL in DIMES as well? DIMES cannot include the "/pdf/" route as part of the setting because it also needs the "/manifest/" route. Is there value in aligning ASSET_BASEURL from pisces with REACT_APP_S3_BASEURL in DIMES?