Allow custom fields to be added to key tables

[plbowers]

Allow an administrator to add custom fields with a limited set of types and they get stored in custom fields which are already included in these key tables.

Thus US would distribute with users.custom1, users.custom2, users.custom3 as unused but available fields

A separate table "custom_fields" would allow choosing a table and a label for these custom fields.

In the form(s) after the normal fields are added there would be a generic "add_custom_fields('users')" or something like that which would read this table and set things up for editing/validating/etc.