Jobs management application/UI

The current Jobs Module is creating background jobs that execute in grouped/ungrouped mode, can be canceled, produce a lot of logs and status files, etc. but none of that is easily manageable or debuggable by an admin.

Some example issues:

• the log files are (almost) unreadable in plaintext and, at times, they can get so large as to risk running out of space.
• the jobs themselves can get stuck or can turn out to take an unexpected amount of time, potentially being very taxing on the CPU, without the admin being able to get an overview of what is going on or any hint of which one might be the problematic job.

To help with this, some jobs related operations should be exposed in the UI, accessible to admins and get a better grip of their XWiki instance:

1. See job execution history:
   • job status and job logs in a human readable / user friendly display, including size on disk,
   • ability to delete old job logs and statuses to clean up and reclaim storage.
2. See actively running jobs:
   • list all running jobs (also see the current hackish alternative to display active jobs: https://extensions.xwiki.org/xwiki/bin/view/Extension/Active%20Jobs%20Application/),
   • read their status and log,
   • be able to cancel cancelable jobs,
   • (bonus/future) start a new job?,
   • etc.