REGISTRAR, GUI: support for new tags in notifications

- 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.

- Old attributes are still supported, but will be probably removed in future.
  Once is this merged, rights for new attribute must be set to r/w for VO ADMIN
  for both and r/w for GROUP ADMIN of group one's.
  
- Removed creation of attribute validationUrl from Registrar.
- Removed support for it's tag in Registrar notifications.
- Removed tag from help in GUI.
- Fixed help text in GUI.
- Code cleanup.

- Once is this merged, all {validationLinkCustom} tags must be removed from
  notification templates - currently VO MU-service and eduGAIN Group Service Demo.
  It can be replaced by setting new attribute and using {validationLink}.
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```