This patch introduces a way to define SMTP servers either globally or
per-library.
To test:
1. Apply this patch
2. Find the SMTP servers entry in the admin page
3. Play with adding/removing SMTP servers
=> SUCCESS: All works as expected
4. Sign off :-D
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com> Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com> Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>