Request: Remove formatting from notifications

[houserx-jmcc]

Describe the bug
For example, while the "xx is down" notification text works fine in GitHub issues, it doesn't format properly in Slack (and I imagine some of the other alert destination types too).

To Reproduce
Steps to reproduce the behavior:

1. Setup Slack notifications
2. Choose a non-existent site to monitor
3. Watch for the alert message

Expected behavior
I think the formatting is fairly minimal and if it was just removed to be consistent across the board it would be an improvement, since many different chat tools do formatting differently.

Screenshots

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I am happy to make a PR if this is an acceptable change!

[AnandChowdhary]

You're right, happy to accept a PR!