Send valid user model to back end

- Add proxy to User model
- Add #save to User controller
- Move account balance validation into separately loaded class
- Add converter openbalance -> accounts
- Refactor fixtures
- Clear Model cache after each test

[#47850963]
User can create a new account

- Add success & failure callbacks to user#save in User controller
- Refactored signup form helper in Siesta

[#47850963]