OPTION: Block unpopular accounts (< 15 followers)

Introduce `block_low_followers` option; implementation similar to `block_new_accounts`
Most changes are in stream.js: bring sequelize object in for .and/.or functions, more trace detail, main logic.
Migration script added for updating `BtUsers` table in existing database.
Non-customizable implementation of Issue #39
Can easily rebase on top of #57 if accepted