Notify reporter/interested parties when an issues has a fix

[dagwieers]

We could notify all involved parties of an issue when a pull request was made that fixes a specific issue.

So if PR X reports it fixes issues Y, the reporter and all interested parties of issue Y could be notified in PR X to get the maximum attention to help test a fix. The notification could actually mention that it speeds up fixing your issue if you take the time to review and report back on the proposed pull request.

[mkrizek]

I'll link #924 here as it is slightly related.