Add optional filtering for sad or disturbing content

[mfitz]

Sometimes we see messages, in particular from dog owners, about their deceased pets. Although it is touching that people want to memorialise their pets in this way, some users find this type of message a little bit of a downer. We should provide a way to optionally filter out these messages, maybe using an env var to switch the filter on or off.

There are several Python natural language processing and sentiment analysis libraries that could help with identifying "sad" messages.