admin/deposit: Use new deposit private API endpoint to list deposits
Previously, the admin-deposit-list view was retrieving all deposits data using the /1/private/deposits/ swh-deposit endpoint and was filtering those client side.
That approach was not efficient at all as the number of deposits increased.
So from now on, swh-web will simply forward the listing requests to the new /1/private/deposits/datatables/ endpoint of swh-deposit which will perform the same filtering and pagination operations as before in an efficient way.
Also remove code that is now dead and ensure deposit private API base URL can be set in configuration with or without trailing slash.
Related to T3128
Migrated from D7550 (view on Phabricator)