This is related to D691#13273 and this ticket. Let's create a FMN rule which will allow users to filter out changed outcomes. This way even if users choose to get notified only about particular outcome (e.g. FAILED), they can still add a new filter with this rule and receive also useful outcome change events (FAILED -> PASSED). Let's name the rule e.g. //Taskotron task outcome changed//. Let's describe that this is useful when an object (a build, an update, etc) gets retested and either the object itself or the environment changes and the task outcome is now different.

Hint: Our FMN rules are here:
https://github.com/fedora-infra/fmn.rules/blob/develop/fmn/rules/taskotron.py