🗃️ Add an arbitrary limit on assets that can be placed in a tiled collection

[Kezzsim]

Resolves #692 by adding a settable environment variable TILED_ASSET_LIMIT with a default value of 100000.
This is to speed up non-paginated loding tasks.

TODO:

• Add SQlite Trigger
• Add Postgres Trigger
• Verify functionality
• create pytest

Please note, if there are multiple concurrent transactions attempting inserts simultaneously there is a chance that a number of records exceeding the maximum may be added. It is widely considered impossible to set a hard limit using a trigger.

That being said, going 1-5 documents over such an arbitrary limit should be acceptable.