Add a config option to filter out low quality messages

[shalzz]

Right now "Low quality" DM's are bridge to matrix as normal private chats and only a warning similar to
"This is a message request. Replying here will accept the request. Note: Twitter has marked this as a "low quality" message." is shown in the initial private chat header.

Add an option in the bridge registration config file that to instead disable receiving DM's (and creating portals for those DM's) that are marked as low quality by Twitter.