Tasks: allow for searching of tasks by 'user'
Description of problem:
It is not currently possible to search for tasks using the search bar via the 'user' field. If you attempt this, an error is raised.
Version-Release number of selected component (if applicable): 6.2.10
How reproducible: every time
Steps to Reproduce:
1. go to /foreman_tasks/tasks page
2. put 'user' in the search box, nothing auto-fills
3. type 'user = admin'
Actual results: error page saying that 'user' is not a valid search term
Expected results: successful search
Additional info: it's useful to search tasks by users to find syncs that were kicked off by a user at a particular time.