The reap function culls expired pastes outside of the process serving
the pastes. Previously the database could accumulate a large number of
pastes and while they were expired they would not be deleted unless
accessed from the frontend.
Individual settings would previously overwrite the whole config, but
now individual values can be overwritten.
Fix missing slash to make the database path an absolute path per
https://docs.sqlalchemy.org/en/14/core/engines.html#sqlite.
Drop preferred_lexers, it's not set to anything meaningful anyway.