Bug 26526: Relabel checkout note column in OPAC

Bug 26526: Relabel checkout note column in OPAC

When checkout notes are active, there is currently a new
column labelled 'Note' in the checkouts table in OPAC.
When a note is added, this is sent to the library. For
the user it's not clear from the label, that these notes
are not personal, but for reporting to the library.
The new label "Report a problem" aims to make this clearer.

To test:
- Turn on AllowCheckoutNotes system preference
- Check out some items to your account
- Go to the OPAC
- Look at the list of checkouts
- Apply patch
- Reload the page and check the checkouts table again
- Verify the heading has changed from "Notes" to "Report a problem"
- Hover over the heading with your cursor to see some more info

Bonus: Patch adds an id "checkout-notes" to the header cell, that
will make it easier for libraries to change text or style if needed.

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>