Optimize `sortKeys` selector

This was recomputing frequently even though the result only changes when
the selected tab changes.

src/sidebar/store/modules/selection.js

@@ -362,16 +362,19 @@ function sortKey(state) {
 /**
  * Retrieve applicable sort options for the currently-selected tab.
  *
- * @return {string[]}
+ * @type {(state: any) => string[]}
  */
-function sortKeys(state) {
+const sortKeys = createSelector(
+  state => state.selectedTab,
+  selectedTab => {
     const sortKeysForTab = ['Newest', 'Oldest'];
-  if (state.selectedTab !== uiConstants.TAB_NOTES) {
+    if (selectedTab !== uiConstants.TAB_NOTES) {
       // Location is inapplicable to Notes tab
       sortKeysForTab.push('Location');
     }
     return sortKeysForTab;
-}
+  }
+);
 
 /**
  * @typedef SelectionStore