Change iiif_presentation_url to iiif_partner_url; Add iif_manifest_url

Note: iif_manifest_url will be uneditable. It will represent the final iiif manifest url that will be used by Mirador and dpla exports when we are ready.

The logic for iiif_manifest_url will be as follows:

1. If there is a iiif_partner_url and iiif_partner_url_enabled is true, use iiif_partner_url
2. Else if there are pages/iiif_images associated with the Item, use "https://dlg.usg.edu/record/{record_id}/presentation/manifest.json
3. If the first 2 conditions are not met, iiif_manifest_url will be blank