This PR adds potentially stale events as available to all addons and
adds a formatted message.
## Discussion
I'd still be interested in hearing input on whether the event type
should be selectable by addons or not before we start emitting the
event. I'm leaning towards "yes", but I'll take your thoughts into
consideration.
## About the changes
Adds more specifics to addon notifications when a flexibleRollout is
updated.
- [x] Specific text for all strategy types
- [x] Add constraint differences for all strategy types
<!-- Does it close an issue? Multiple? -->
Closes [#3140](https://github.com/Unleash/unleash/issues/3140)
* fix: slack back link url to point to the feature page
* update snapshots
* update feature link in case project info is available in event
* update missing snapshot
