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
The paging controls -- as far as I can tell -- are being all rendered from
layouts/_pagingcontrols.html from which we call update_paging_url_parms()
to construct the navigation urls.

update_paging_url_parms() simply takes the relate path from request:

    "#{request.path_info}?#{updated_query_string.to_query}"

So what's being called when you hit the navigation buttons depends on the request
that rendered the list view that you're clicking on.

If you initially land on the 'Tasks' page, you're being presented with the 'My VM Analysis Tasks'
tab -- request is by /miq_proxy/index. Paging requests then go to /miq_proxy/index.

If you change the tab to something else ('All Other Tasks' for example), the request changes
to /miq_proxy/change_tab and all subsequent paging request then go to change_tab.