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.
GUI: use new tags in help for registrar notifications
REGISTRAR,GUI: removed support for {validationLinkCustom}

- Removed attribute creation from Registrar.
- Removed support for tag in Registrar notifications.
- Removed tag from help in GUI.
- Fixed help text in GUI.
- Code cleanup.
REGISTRAR: code optimization

- Unified shared code into private methods.
- Public methods moved above private.
- Updated javadoc.
GUI: code optimization

- Fixed "<br/></br>" converted to <p>.
Merge pull request #571 from zlamalp/registrar

REGISTRAR, GUI: support for new tags in notifications