Add support for "is row checkable" in grids

i.e. when grid has checkboxes, some rows maybe shouldn't get one

tailbone/templates/grids/buefy.mako

@@ -192,12 +192,17 @@
        % if grid.check_all_handler:
        @check-all="${grid.check_all_handler}"
        % endif
-       ## TODO: definitely will be wanting this...
-       ## :is-row-checkable=""
+       % if isinstance(grid.checkable, six.string_types):
+       :is-row-checkable="${grid.row_checkable}"
+       % elif grid.checkable:
+       :is-row-checkable="row => row._checkable"
+       % endif
 
+       % if grid.sortable:
        :default-sort="[sortField, sortOrder]"
        backend-sorting
        @sort="onSort"
+       % endif
 
        :paginated="paginated"
        :per-page="perPage"