UI Tasks view: fix per page settings

Configuration -> Tasks -> per page settings and page navigation
wouldn't work correctly:
* the view would always default to 50 items per page
* incorrect numbers shown next to the navigation icons

Fixes #818

https://bugzilla.redhat.com/show_bug.cgi?id=1171150
@mzazrivec  I think a fix might be needed in ```update_paging_url_parms```.
There are essentially two parts to this method - the path and the url params.
The path is what needs to be addressed. Let me get back to you on what could be done to fix the path.