[ENHANCEMENT] Streamline/standardise externalDatabase configuration across identity and webmodeler

[bkenez]

Describe the use case:

identity and webmodeler's externalDatabase configurations are not consistent with one another.

It has been suggested before that a JDBC url should be the one standardised way to set this.

Describe the enhancement/feature:

Rely on a single URL field to configure external databases, deprecating individual host/port/db values.

Desired outcome and acceptance tests:

Appropriate values added, deprecated, and validated with unit tests and constraints.