Allow profile fields to be configured with a default

No guarantees I will take this on but recording it

DGG:
The general enhancement of allowing default values to be set for profile fields has come up in the past, and seems useful - ... adding a 'default_value' column to civicrm_uf_field seems consistent (that's the column name in civicrm_custom_field). I think this property would need to inherit it's default default_value from the referenced core or custom field if they have one set. This happens automagically now for custom fields with default values - so we would need to think about the inheritance rules. For example, if I set 'Married' as the default value for the sample custom field "Marital Status" AND I add that custom field to a profile ... how should we handle default setting in the profile if civicrm_uf_field.default_value is NULL?

Me:
Note that I also think that we should add an 'id_hidden' column. Any field with a default should also be able to be hidden so that the default is simply submitted.