Bug 26569: Use gender neutral pronouns in system prefernece explanations

Bug 26569: Use gender neutral pronouns in system prefernece explanations

To test:
1 - perform query "select variable,explanation from systempreferences where explanation like '% his %';"
2 - find 6 sysprefs with gendered pronouns used in explanations
3 - apply patch, reset_all
4 - repeat query
5 - no more gendered pronouns!

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>