Bug 20911: Use 'POST' to submit search history forms

Bug 20911: Use 'POST' to submit search history forms

To test:
1 - Enable EnableOpacSearchHistory and EnableSearchHistory
2 - Do some searches
3 - View your search history in staff/opac
4 - Confirm you can delete single or multiple rows
5 - Confirm other page functionality has not changed

Signed-off-by: Ed Veal <eveal@mckinneytexas.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>