Consider alternatives to modoffensive

[duogenesis]

I don't like how inaccurate it is. It filters offensive messages too indiscriminately. Jokes that are "offensive" in some sense might still be wanted by the recipient because, for example, the recipient has an edgy sense of humour. However, very few people want to receive, for example, insults. So it should filter insults without filtering offensive jokes.

It might be possible to remove modoffensive if there's a different set of restrictions placed on users. For example, if new users can only send a small number of messages until they've increased their reply rate to a certain value. Then again, offensive messages often provoke the recipient to reply...

Related: #537 #535