Add referese code as an parameter needed to registed a user and thus a domain

[gruz]

If reference code need is enabled in configuration, then user creation data must provide such a reference code (a code from a billing system to track calls to world) and the code must match a list of available codes.

The list of the codes will be taken from an external api in the future. For now this can be a table model.

Since we have already added email_verified_at column to v_users table anyway, we may add the reference code column as well.

Or maybe move these fields to a separate table not to intrude into FPBX tables. This seems to be a better option.