If a user selects a message prefix, then the system should fetch the status of all messages with that prefix and display their status in the same format as the `Special:MessageGroupStats` currently shows for messages groups. This information will have to be computed on the fly.
The `EntitySearch` class can be used to identify all the messages that have that prefix. To determine the status, we would have to hit the database like `MessageCollection` class does to identify reviewed or translated messages.
Some other notes / thoughts on the implementation:
* We should also enforce some limit to ensure we don't allow too slow queries to overload the server.
* A short duration cache of the results may or or may not be useful to reduce load in case a lot of people are looking at the same thing at the same time (someone sharing a link to this stats page for example)
* We may also need to refactor some code in `MessageGroupStats` to avoid code duplication when calculating the stats. This needs to be evaluated how much can be re-used sensibly.
* One (possibly ugly, though) idea is to pass a fake group that includes just the keys provided, and run that query without storing results in the DB