Improve resource syncing (refactor + handle mapping cleanup)

[lukinovec]

This PR adds listeners for handling cleanup of pivot records/mappings

• when any synced resource got deleted (previously, the cleanup was only happening on SyncMaster/central resource deletion) - on SyncedResourceDeleted (fired when any synced resource got deleted), DeleteResourceMapping
• when a tenant gets deleted - on TenantDeleted, DeleteAllTenantMappings (until now, the pivot records were only getting deleted when the tenant key column had a constraint, and the constraint had onDelete('cascade') -- db-level)

This PR also improves some resource syncing implementation details, for example, the ResourceSyncing trait now uses static::deleted() instead of static::deleting(), which is safer. The SyncedResourceDeleted event with the DeleteResourceMapping listener also lets us separate the resource x pivot record deletion concerns, and improve the performance (e.g. the mass delete of pivot records when SyncMaster's deleted instead of a for loop where we were deleting the pivot records one by one).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.56%. Comparing base (8bdacf5) to head (728d2db).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1411      +/-   ##
============================================
+ Coverage     85.46%   85.56%   +0.09%     
- Complexity     1133     1140       +7     
============================================
  Files           181      184       +3     
  Lines          3323     3345      +22     
============================================
+ Hits           2840     2862      +22     
  Misses          483      483              

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