[subscriber] Producer capability from subscriber!

Gigantic commit. Holy crap. Anyway,

+ Subscriber users can now add producer plugins that format and submit
  jobs right from the producer automatically.
+ New task wizard overhaul. Includes aforementioned functionality.
+ API for subscriber plugins. This will eventually include more than
  just producers.
+ Subscriber becomes the 'main' user interface for most users, as it
  probably should be.

- me.devyn.maru.Echo subscriber plugin not yet implemented.
- Subscriber plugin API is incomplete; it is the minimum required to get
  org.blender.Render working.
- User experience still kinda sucks.

Planned changes:

+ With a configuration option, allow users to upload prerequisites
  directly to the subscriber. This will not affect the existing plugin
  API (yay!)
+ Multi-user support in the subscriber. I don't expect everyone to be
  able to run their own, even if it does help distribute load, so this
  is a compromise, and these latest features are very personal anyway.
+ Keep track of jobs that the subscriber has produced in the DB.
+ Allow producers (including the subscriber) to monitor status of the
  jobs they have submitted on a network. Probably a watch command of
  some sort.

- Stop using localStorage for task information