Bug 19855: Move getalert, addalert and delalert to Koha::Subscription

Bug 19855: Move getalert, addalert and delalert to Koha::Subscription

This patch removes 3 subroutines from C4::Letters:
- getalert
- addalert
- delalert

And add 3 methods to Koha::Subscription:
- subscribers
- add_subscriber
- remove_subscriber

It makes the code cleaner for future cleanup.
TODO - we should remove alert.alertid and alert.type, and rename
alert.externalid with alert.subscriptionid
That way alert will be renamed borrowers_subscriptions (or similar) and
will become a simple join table between borrowers and subscriptions.
We will need to deal with FK that could not be satisfied.
Let's do that after this patch is pushed.

Test plan:
Subscribe and unsubscribe to email notifications sent when a new issues
is available.
Make sure everything works as before and you receive the emails.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>