Implement new password policy

🗣 Description

Add new password policy to the Rego (i.e., min password length of 15).

Closes #475.

Modified unit tests
Manually tested, both on top-level OU and a child OU, including setting inheritance.

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[x] Tests have been added and/or modified to cover the changes in this PR.
[x] All new and existing tests pass.

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