Auto-post category advice on uncategorized help threads

[Zabuzard]

Sometimes new users create a help thread and then dont select any category at all.

In such a situation, the bot should (after some waiting time, maybe 5 minutes) post a message where it pings the author and asks them to select a category, shortly explaining how the system works.

This could be setup as scheduled task, scheduled by ImplictAskListener.java and AskCommand.java (via a separate helper class). Where we could schedule this "explain-job" in 5 minutes after creating the threads. The job would then check the thread history to see if there are any messages already (other than from the bot) - and if not, it would post this message.

Also, we would have to setup a small database to memorize help thread channel ID <-> author ID in order to be able to know who to ping.

(This is similar to #481 )