Use custom properties to enable/disable alerts per reload task

[mountaindude]

Overview

1. New config settings to control if alerting should be enabled for all/some reload tasks
   1. Independent config for each notification channel (Teams, Slack, email, incident mgmt tool, webhook)
2. For email notification channel: All alerts sent to all users defined in config file, or per-reload-task recipients.
   1. Define a list of recipients that should get all alerts (as done in current Butler version) for all alert-enabled reload tasks.
   2. A custom property (on reload task) that contains the email address of the user(s) that should get alerts.

For email notification channel:

Two new ways to control which failed or aborted reload tasks result in alert emails:

1. A custom property is used to control whether ALL failed reload tasks should result in alert emails, or only some
   1. The custom property name is configurable in Butler's config file
   2. The exact custom property value used to enable per-task alerts is also configurable in Butler's config file
2. Another custom property is used to specify, on a reload task level, which email addresses should receive failed reload tasks alert emails.
3. All ways of specifying who should get alert emails are cumulative, but before alert emails are sent a de-duplication is done to prevent more than one email being sent to any email address

The above is also implemented for aborted reloads, with its own set of custom properties specified in the config file.