webadmin: Fix server-side sort issue

This patch fixes an issue where triggering server-side sorting
on a given column (via mouse click on sortable column header)
might corrupt the actual search query dispatched by the model.

- UI search string contains syntax error(s)
- UI search string contains SORTBY syntax object

If any of above holds true, sorting should be cleared across
all columns of the given table - all columns should appear as
not sorted.

After applying this patch, following behavior is enforced:

a, assume there is NO sorting applied on the given table, user
   triggers server-side sorting on a given column:
   a1, if the UI search string is 'valid' (no syntax errors,
       no SORTBY present), append SORTBY to the search string
       when dispatching actual search query (without updating
       UI search string) and execute refresh to fetch items
   a2, otherwise, clear sorting on given table - all columns
       should appear as not sorted

b, assume the given table is sorted on a given column, user
   enters new UI search string and applies it via Search icon
   (or by pressing Enter key):
   b1, if the UI search string is NOT 'valid' (syntax errors
       and/or SORTBY present), clear sorting on given table
       - all columns should appear as not sorted
   b2, otherwise, proceed with applying sorting on the given
       column

Change-Id: I7ce39a4e2b6323e6e0276a3a462a3f9da005344b
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>