[Feature]: Add Maintenance Mode

[scottdykes1611]

Product

None

Is your feature request related to a problem? Please describe.

When deploying a new release, sometimes everything is broken for a while. This should be shown to be "expected behaviour" to a customer.

Describe the solution you'd like.

Add some popup on login/dashboard page: "Maintenance in progress"

Describe alternatives you've considered.

No response

Checked for existing ticket

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