Allow CodeyBot to generate random phrase based on user messages (with consent) and Markov chain

[KuroganeToyama]

Summary of Changes

• This allows a user to consent to have their messages read, to generate random phrases from any other consented users, and to quit from the activity.
• At 0AM of each day, new messages from consented users will be collected to update the database.
• At 0AM on the first day of every month, any data older than 1 year old will be cleared from database to avoid blowup.

Related Issues

Issue #543

Steps to Reproduce

• Run CodeyBot as usual, a table phrase_users and phrase_messages will be created in the SQLite database.
• /phrase signup will attempt to sign the invoker up and collect their messages in the server within 1 year, or tell invoker that they are already signed up. If it's signing a new user up, it will take several minutes to finish.
• /phrase create or /phrase will generate a random phrase based on the invoker.
• /phrase create @username will generate a random phrase based on the target user, if consented, otherwise it informs that the target user has not given consent.
• /phrase quit will remove the invoker from the database and wipe out all message data related to the invoker.