REGISTRAR: removed usage of vo/group appFormUrl attribute

- Removed usage of appFormUrl attribute for {invitationLink[Authz]} tag.
  Value is taken from perun configuration. For custom URLs now
  can be used new attribute registrarURL and {invitationLink} tag.
REGISTRAR: fixed invitation link replacement
Merge pull request #573 from zlamalp/registrar

REGISTRAR: removed usage of vo/group appFormUrl attribute