Syncing: SyncedResourceDeleted event and DeleteResourceMapping listener

Also move pivot record deletion to that listener and improve tests

The 'tenant pivot records are deleted along with the tenants to which
they belong to' test is failing in this commit -- the listener
for deleting mappings when a *tenant* is deleted is only implemented
in the next commit. The only change done here is to re-add FKs
(necessary for passing *in this commit* in that specific dataset
variant) that were removed from the default test migration as we now
have the the DeleteResourceMapping listener that's enabled by default.

src/ResourceSyncing/Syncable.php

@@ -16,6 +16,8 @@ interface Syncable
 
     public function triggerSyncEvent(): void;
 
+    public function triggerDeleteEvent(bool $forceDelete = false): void;
+
     /**
      * Get the attributes used for creating the *other* model (i.e. tenant if this is the central one, and central if this is the tenant one).
      *