Show planned maintenance windows in dashboard

[mbernasocchi]

Is your feature request related to a problem? Please describe.
we're very happy with the new dashboards, which have become integral to our communication (https://status.qfield.cloud/). There is, unfortunately, a big shortcoming for us.
A planned maintenance window is only visible once it becomes active.

To really become fully operational as a status page, these should be visible ahead of time.

Describe the solution you'd like
show planned maintenance as non-active planned incidents

Describe alternatives you've considered
A minimal solution could be to show planned maintenance as in the admin dashboard

Additional context
potentially part of #242 and #240 and #65

[tnolet]

@mbernasocchi thanks for reporting! Taking notice for the next iteration.