Makes most tasks no longer UI-blocking, allows to cancel tasks, adds progress bars for them and much more.

Depends on #25 and #29.

Things that still need to be done:

• Check for potential race conditions, and if it's possible for async automatch tasks to override user matches
• Extract Job4j into its own repo
• Fix main progress bar flashes when new jobs get started
• Allow jobs to pass a message as to what's currently being worked on
• Allow job titles/text to be translated
• Fix progress count sometimes not updating all the way to 100% (being stuck at 99%)
• Investigate a rare issue with cancel buttons not displaying the correct state:
  Buttons with wrong state