Bug 29989: Improve headings in MARC staging template

Bug 29989: Improve headings in MARC staging template

This patch updates the MARC staging page so that the headings are
descriptive based on the context.

To test, apply the patch and go to Tools -> Stage MARC for import.

- The heading should be "Stage MARC records for import."
- Upload a MARC file and stage the file for import.
- The heading on the next page should read "MARC staging results."

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>