#4361 - add SUPPORT_MESSAGE configuration setting

[glennmatthews]

Closes: #4361 (will need double-commit to ltm/1.6)

What's Changed

Add a new config setting SUPPORT_MESSAGE that can be configured in nautobot_config.py or via Constance. When set, this message (Markdown format) will be rendered into any 4xx or 5xx page returned by Nautobot, replacing the standard message about joining NTC's Slack community and posting in #nautobot.

Additionally, fixed a couple of bugs in the 2.0 documentation that I noticed along the way.

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• Attached Screenshots, Payload Example
• Unit, Integration Tests
• Documentation Updates (when adding/changing features)
• n/a Example Plugin Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design