REGISTRAR: new version of tag replacements

- Registrar creates and uses new attribute holding base URL of Perun instance
  for VOs and groups. It's value is used in new tag replacements.

  If attribute is not set (for group) use VOs as backup.
  If attribute not set either (for VO) then use config property "perunUrl".

  If value can't be determined, replace tag by empty space.

- New tags look-like: {tagName-authz} or {tagName} when only attribute value
  is used. If authz is provided, it's appended after attribute value.
  Then, rest of address is calculated from provided context params.