Add contract date related notifications

[mlebreuil]

Add the possibility to be notified at contract renewal

[mlebreuil]

Netbox does provide starting with version 4.1. a notification mechanism which can be based on event rules.
Currently, however, the simplest solution is to use a custom script.
An example is provided under the utils folder.

TODO:

• Document the use of Event Rules and Notifications.
• Revisit the use of Jobs instead of custom scripts.