Bundle User and UserPrivilegeRuleset into a single transaction upon creation

[SaracenOne]

Currently the UserPrivilegeRuleset for a User is created after the initial database creation due to creating a User being handled by an opaque function provided by the Pow library and could theoretically result in a User being left without a valid ruleset. The correct behaviour would be to create both database entries as part of a single transaction, but it is unclear how to do this.