Bug 13557: Add hint for on-site checkouts to list of current checkouts in OPAC

Bug 13557: Add hint for on-site checkouts to list of current checkouts in OPAC

In the list of checkouts it's currently not possible for the
patront or tell the difference between normal checkouts (item
is at home with them) and on-site checouts (items remained at
the library). This patch adds a hint to the list of checkouts to
allow differentiating these kinds of checkouts.

To test:
- In your patron account, have multiple checkouts, some on-site,
  some others, overdue items, etc.
- Go to: OPAC > your summary > Checked out
- Verify that there is no hint for the on-site checkouts
- Apply patch
- Check again, there should now be a note

The note also has a class, so would be easy to be hidden
or formatted differently: class="onsite_checkout".

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>