Better and more customizable notifications

DebugBear automatically generates notifications if it looks like there's been a regression on your site. That means you don't need to do any work to get set up, but you might get some notifications that aren't relevant to you.

From now on you can configure when a notification is sent. It's been possible to mute specific notifications for a while, but I've now added some documentation for it.

One common issue has been notifications for performance issues that can't be reproduced later on. Maybe the server was busy, or something weird happened with DNS. To avoid this problem in the future, some performance alerts are now only sent if the performance problem occurs more than once in a row.