Assorted changes

There are now four categories of jobs:

- Enqueued: any job that has not failed, is not being worked on and is not
  scheduled in the future.
- Working: any job that has been locked for processing.
- Failed: any job that has failed a number of times equal to the maximum
  attempts setting.
- Scheduled: any job that is scheduled to be run at a time in the future.

Removed the redundant stats page.

Rationalised the actions into four:

- Delete a specific job
- Reload a specific job
- Delete all failed jobs
- Reload all failed jobs

Reloading means that the job will be enqueued immediately, and all failed
attempts and error information will be reset.