RPC: fixed RPC javadoc
REGISTRAR: support for custom validation links

- Allow to use custom email validation link. Base URL is taken from
  new VO or Group attribute (vo as backup for group).

- If provided custom link is not valid (empty) registrar will
  try to use non-authz link as backup value.

- Registrar creates new attributes. After deploy, privileges must be set
  to both: R/W: VO admin and to group attribute also R/W: Group admin.

- Mail manager now doesn't send not valid URLs (empty base url but  with params).
Merge pull request #526 from zlamalp/validation

REGISTRAR: support for custom validation links
GUI: display URN on hover above attribute friendly name cell
Merge pull request #527 from zlamalp/urn

GUI: display URN on hover above attribute friendly name cell
Merge pull request #524 from zlamalp/docs

RPC: fixed RPC javadoc
GUI: added {validationLinkCustom} to help for notifications
Merge pull request #528 from zlamalp/custom

GUI: added {validationLinkCustom} to help for notifications
GUI: better display of user's name from cert DN

- Display readable CN from cert DN if user is unknown
  and on submitted applications page.
Merge pull request #529 from zlamalp/cert

GUI: better display of user's name from cert DN
Fixed name of the default Perun conf. directory
Added getHostsByHostname function.
Merge pull request #531 from CESNET/getHostsByHostname

Added getHostsByHostname function.