External Requisition UI: Thread pools adjust upper bound validation

The External Requisition UI enforces thread pool limits of 1 .. 2000 for all entries.

This value is too high for import and write as typical sane values are:

Scan and rescan: 10 (default) – 1000 (or higher as one customer uses ~1800)
Import and write: 8 (default) – 100

From jeffg
Let's adjust the upper bounds to:

Scan and rescan: 2000 (***already the case)
Import and write: 100

I'm totally fine with the REST API not enforcing any limits. This gives advanced users an option for pushing it beyond "manufacturer's spec".

Snackbar new error message: Thread pool values are outside of supported range.