Subtasks

The idea here is to let any plugin that allows entry generation via input subplugins, (discover and configure_series,) have a more general way to get their input, that works like a full task (with manipulation and filtering.)

Possible TODOs/concerns:
- Make a new option for tasks, where every entry starts already accepted, rather than hacking in accept_all
- Right now, it makes and commits a new sqlalchemy session. Should it?
- requests session should maybe be transferred from main task?