Relationships should only be loaded where necessary

I noticed a few problems in our controller code:

• Edit pages only allow editing relationships where the source entity cardinality is 0, but we load all relationships anyway (which can be very slow on some classical artists).
• The tabs for aliases, tags, details, etc. only display URL relationships in the sidebar, but in some cases we're still loading all relationships for the entity on those tabs.
• Entities that use Controller::Role::Relationship load URL relationships twice on the edit page.