Modules that set messages aren't shown

We have a clear pathway to doing this now.

In InstallerController::activate(), as we build the AJAX response, we could pull all messages out of the messenger service and add them to the response with the MessageCommand.

To test it, all we'd need to do is make TestActivator set a message or two in its activate() method when handing a specific (fake) extension, and then the test would just confirm that those messages show up on the frontend when that fake extension is activated.

Assigning to @kunal.sachdev for now.

kunal.sachdev → made their first commit to this issue’s fork.

This seems really good and straightforward. A few relatively minor suggestions.