REQ01 - Improvement - display only custom fields with type that can be managed

[fmancardi]

Current implementation (on original repo), when accessing the page used to configure link between Custom Fields (CF), display ALL AVAILABLE CF, but because only CF of type: Enumeration OR Multiselection list can be used, we can have lot of CF that can not be used, and for each CF on the table the message 'Only supported for fields of type Enumeration, Multiselection list' is displayed

A change is going to be made, in order to

1. give JUST ONCE the message 'Only supported for fields of type Enumeration, Multiselection list'
2. display only CF that can be used on this feature