Revert "Convert UserType to an enum class".•••There's a bug in GCC with bitfields and enum class.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51242
This reverts commit 44834a66a7e92c8979ae16421286c581a9cffbe9.
Implement serialisation of users.•••This allows for various things which will be coming in the future.
e.g. Transferring users to another server on upgrade.