Always initialize opt-in ping list

If OPT_IN_PING is enabled via botconfig.py, OPT_IN_PING isn't set to
True at the time we set up tables as necessary, so people using !in will
make the bot run into the lack of table and crash.

We already do this with the away list even if we're going to use opt-in
ping, so this shouldn't cause any issues.
Move database/persistence initialization into a function

This allows us to postpone init_db(). One advantage of this is that
specifying AWAY in botconfig.py won't overwrite the away list from the
DB anymore, but properly get added to (as would be expected).