Disable aliases when an object is deleted or unshared

[jportner]

Kibana version: master (pre-8.0 release)

Describe the bug:

Legacy URL aliases should only exist when they have a valid target to point to.
Today, if an object is deleted, or if it is unshared from a space, the alias will be unaffected. This doesn't affect users in 7.16 but it can in 8.0.

Scenario:

1. Space X has an alias "foo:123" that points to object "foo:456"
2. User deletes "foo:456"
3. User attempts to create "foo:123", this will result in an alias conflict error, even though that operation is not actually problematic because the alias has no valid target

Expected behavior:

When an object is removed from a space in any way, if it has an alias pointing to it, that alias should be deleted or disabled.
Disabling it would be the safer bet in case users want to revert to their previous state, but that would mean the only way users could fully delete aliases organically by using the Kibana UI would be to delete an entire space. Maybe that's acceptable and we can add some sort of alias cleanup task in the upgrade assistant or an alias management UI in the future to allow users to remove stale/disabled aliases.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)