Only toggle item's select status in multiselect (Graylog2#4715)

- On a single select, clicking on a selected option will not clear the
  selected item
- On a multiselect, clicking on a selected option will deselect it,
  while keeping the status of other selected items

graylog2-web-interface/src/components/common/SelectPopover.jsx

@@ -100,10 +100,11 @@ const SelectPopover = createReactClass({
     return () => {
       const selectedItems = this.state.selectedItems;
       let nextSelectedItems;
-      if (selectedItems.includes(item)) {
-        nextSelectedItems = lodash.without(selectedItems, item);
+      if (this.props.multiple) {
+        // Clicking on a selected value on a multiselect input will toggle the item's select status
+        nextSelectedItems = selectedItems.includes(item) ? lodash.without(selectedItems, item) : lodash.concat(selectedItems, item);
       } else {
-        nextSelectedItems = this.props.multiple ? lodash.concat(selectedItems, item) : [item];
+        nextSelectedItems = [item];
       }
       this.handleSelectionChange(nextSelectedItems);
     };