improve user privacy

[zephyrtronium]

• Add a "privacy" privilege level. Users of this level can trigger commands, but the bot does not record any information from them, in history or in tuples.
• Add a "priv" channel config option, setting the default privilege level for each channel. Usually this will be null or "privacy", depending on the broadcaster's wishes.
• Add a "privacy" command. Responds with a link to a summary of the information Robot collects (like in the current top-level README) and mentions commands to manage a user's own privacy.
• Add "ignore me" and "unignore me" commands. The former sets a user's privileges to "privacy", or to "bot" if they're currently an admin. The latter sets a user's privileges to regular, or to "admin" if they're currently "bot".
• Save hashes of usernames in history instead of full usernames.
• Remove mentions of users with "privacy" privileges when learning chains. Replace with self, triggering user, broadcaster?