Adding maintenance reminders.

[ToxykAuBleu]

This pull request is an implementation of #428. It allows users to send reminders when a maintenance is about to start and end + before x amount of time. It adds one more option on Subscription page: Send Reminders Maintenance.

If for some odd reasons, the user updates start or end date time, depending on current time, reminders will not be sent twice. This is also the case when your kener instance is stopped during a maintenance.

Each mail contains a clear description of upcoming maintenance, such as name of maintenance, starting date, duration, and a description of affected monitors. The ending reminder contains only name of maintenance, ending date and affected monitors
Mails were tested on Gmail, iPhone (Mail app) and Outlook.

Changes:

• Updating incidents table.
  • Adding 20250529122728_add_reminder_column.js migration.
• Adding 2 new options related to reminders on Incident creation page.
• Adding new email template: maintenance_reminder.html
• Adding documentation for incident management.
• Adding corresponding checks in startup.js

Previews:

Reminders' options:

Example of mail > upcoming maintenance:

[Scholdan]

Could you merge and release this? :)

[yevhenk86]

Does it refer only to maintenance? It would be great to notify users about incidents too.

[Scholdan]

Could this be merged?

[Scholdan]

@ToxykAuBleu maybe you can add so I can add users to the list myself?

[ToxykAuBleu]

@ToxykAuBleu maybe you can add so I can add users to the list myself?

@Scholdan I'm not a maintainer. The only person who can decide if this PR can be merged is @rajnandan1.
Though, I do agree that this PR is stale for almost 3 months 👀

[Scholdan]

@rajnandan1 Can you take a look?

[Scholdan]

@rajnandan1 ?

[ymmijgno]

@rajnandan1 Can we merge this please? This is an awesomely useful feature!

[Scholdan]

@rajnandan1 did you abandon this project?

[rajnandan1]

No. Will be back soon. Lot of backlog.

[rajnandan1]

This request/issue/feature has been added to v4.x
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