Allow to manipulate workspace from the job page

allow to do simple operations on the workspace: delete file, set/clear readonly flag.
These two are all it is really needed for tweaking the workspace for minor failures while building.
For example, TFS has a problem with files without readonly flag, therefore it would be easier to just go to workspace and set the flag (right now I have to do it thru Windows Explorer, and at some point I may not have direct access to that folder).