Bug 28388: Preserve "Browse results" when "view plain" is clicked

Bug 28388: Preserve "Browse results" when "view plain" is clicked

This patch modifies the way search sessions are preserved in the OPAC so
that viewing the "plain MARC" view will not cause the search context to
be lost.

To test, apply the patch and make sure OpacBrowseResults is enabled.

- Perform a search in the OPAC which will return multiple search
  results.
- View the details of one of the search results.
  - You should see a "Browse results" box in the right-hand sidebar.
- Click the "MARC view" link.
  - If you click back to the "Normal view" now, the results browser
    should still appear.
  - From the MARC view, click the "view plain" link.
  - Return to the "Normal view."
    - Before the patch: The results browser is gone.
    - After the patch: The results browser is still there.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>