Bug 28130: Manage subscription alerts on OPAC

Bug 28130: Manage subscription alerts on OPAC

This patch adds an 'Alert subscriptions' tab to the patron's account on the OPAC, so they can easily view or cancel email alerts they have subscribed to.

To test:

1. Subscribe to email alerts for one or more subscriptions via the OPAC
2. Go to your account, notice the new 'Alert subscriptions' menu option, click here
3. Confirm your subscribed alerts show here. Confirm the table sorting works etc (JS enabled).
4. Test unsubscribing from email alerts, make sure the confirmation pop-up works as expected (JS enabled).
5. Confirm that, when unsubscribing, you are redirected back to this page.
6. Confirm the menu option disappears from the left sidebar navigation if you have no alert subscriptions.

Sponsored-by: Bibliotheksservice-Zentrum Baden-Wuerttemberg
Signed-off-by: Christian Stelzenmüller <christian.stelzenmueller@bsz-bw.de>
Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>