Bug 22677: Include hint to OAI page path on OAI-PMH preference

Bug 22677: Include hint to OAI page path on OAI-PMH preference

This copies the way we are doing it for othe rprefs (ILSDI and similar)
and adds a hint to the description to make it easier to find the
right path for the OAI server.

To test:
- Apply patch
- Verify that the system preference for OAI-PMH includes
  now a hint to: /cgi-bin/koha/oai.pl?verb=Identify
- Verify the hint is correct (add OPAC base URL)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>