Refactor user module to use helper functions for adding users and their permissions to the database

[einkoro]

Inserting a new user and their permissions should be split into helper functions and/or prepared queries so other modules can use them.

For example – the cart registers new users when they checkout and then adds their permissions to the database.

[einkoro]

There is already a lot of public functions in the user module and it looks like we just need: set default permissions, insert new user (with and without confirmation required).

...New users should probably set default permissions anyway and we can skip the check every page request currently in users.