Show the scheduled time in the message

Problem/Motivation

My users need to know when the maintenance starts. I would like them to see it in their own time zone, too. Therefore, duplicating the start time manually in my message is not an option.

Steps to reproduce

Schedule maintenance, with a message. The start time of the maintenance window is not shown in the message.

Proposed resolution

Allow using tokens like [start_time] or %startTime in the message. This would solve the issue for all users (not just developers).

AND/OR:

Refactor the `ScheduledMaintenanceSubscriber` class: Split its methods into smaller methods, which would allow developers to extend the class and override some of the methods. E.g., there could be a `getWarningMessage` method that builds the message. A developer could override this to support tokens.

Remaining tasks

T.B.D.

I'm willing to implement this.

User interface changes

Expand the description below the "message" field to tell the user which tokens are available.

API changes

None.

Data model changes

None.