migrate map download table to inline

[adambreznicky]

currently, MSD map collections has map download files maintained in a separate table from the collections themselves, with their foreign key related table being the collection each download belongs to. this should be migrated to an inline form within the admin console like images are to lcd collections.

[adambreznicky]

when making the change, should migrate the deliberate refresh_materialized_views lambda invocations (2 of them) in forms.py over into the admin.py MapCollectionAdmin under the save_formset method like LCD and LORE (reference for how to implement)