Add BANNER_MAINTENANCE environment variable to set the Maintenance Banner

[sweickge]

Resubmitting new PR because of branch issue

New Behavior

Add environment variable (BANNER_MAINTENANCE) to be able to set a custom Maintenance Banner in the docker compose override file

This parameter is described in the Netbox documentation: https://netboxlabs.com/docs/netbox/configuration/miscellaneous/#banner_maintenance

Contrast to Current Behavior

Right now, this can not be set as an env var

Discussion: Benefits and Drawbacks

Right now, the only way to set this would be defining the variable in configuration/extra.py, which would require a rebuild of the netbox-docker image anytime it is changed. An env variable would make it easier to change. This is similar to how how other banner variables are currently defined e.g BANNER_TOP and BANNER_BOTTOM.

If approved, I would like to see this change backported as well to a 4.0.X release since that still supports Netbox 4.5.X if that is possible. As 4.6 is still new, we are unable to upgrade at this time. This change would be backwards compatible to any Netbox release that has the BANNER_MAINTENANCE parameter.

Changes to the Wiki

N/A

Proposed Release Note Entry

Enable setting BANNER_MAINTENANCE parameter as an environment variable

Double Check

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