Show input field after select2 focus

After select2 upgrade input field not always shown when autocomplete
field receives focus. This PR contains changes to source of select2,
that is not a best solution, but otherwise we need either to
update major version of select2(a lot of incompability issues for
extension) or monkey-patch select2.

ckan/public/base/vendor/select2/select2.js

@@ -1989,7 +1989,7 @@ the specific language governing permissions and limitations under the Apache Lic
                 if (data.results.length === 0 && checkFormatter(opts.formatNoMatches, "formatNoMatches")) {
                     render("<li class='select2-no-results'>" + evaluate(opts.formatNoMatches, opts.element, search.val()) + "</li>");
                     if(this.showSearch){
-                        this.showSearch(search.val());
+                        this.showSearch(true);
                     }
                     return;
                 }