Support for summarized and scheduled notifications

[MM-msr]

Description

Currently, Dependency-Track Alerts send a single notification for every single event happening that an alert is subscribed to (e.g. NEW_VULNERABILITY). This can lead to a lot of emails for users and creates the risk of overlooking important notifications.

To improve the user experience of the Alerts, scheduled notifications are implemented in Dependency-Track to send summarized notifications of new events between the last and current scheduled notification. The schedule is defined as cron expression.

Scheduled notification management is available via API in the Backend Pull Request DependencyTrack/dependency-track#3925 and Frontend under Administration -> Notifications -> Scheduled Alerts.

Addressed Issue

DependencyTrack/dependency-track#322

Additional Details

• new category under Administration -> Notifications named Scheduled Alerts
• new column in Administration -> Notifications -> Templates to show, if template is dedicated for scheduled publishing
• available publishers are filtered in dropdown lists while creating new alerts
• option in scheduled alerts to only send a notification, if new subscribed events occurred since the last published notification
• a separate notification is sent for each selected notification group

For screenshots, see backend PR: DependencyTrack/dependency-track#3925

Checklist

• [x] I have read and understand the contributing guidelines
• [ ] This PR introduces new or alters existing behavior, and I have updated the documentation accordingly

[MM-msr]

@nscuro As you may have already discussed with @rkg-mm, i would appreciate it if you would provide a feature branch for this PR.

[nscuro]

Superseded by #1210