Add resource fieldset to a group

Create a resource field for a user group based on following requirements:
- it is a key value pair
- the key can be selected from a pre-defined list
- the value is a free text, it can be an url also, that must displayed
  as a link
- the field can contain multiple elements (0..N)

Change-Id: I76e46882ef734849ee8a957a658a95849cb5f0a5