Improve performance of _getLeftPaneList (#3398)

* Improve efficiency of conversation sorting in getLeftPaneLists

- Sort conversations and archivedConversations separately, without 
  items that had falsey activeAt
- Don't convert conversation titles to lowercase before comparing:
  collator.compare() is case-insensitive anyway.

* Enable caching for format() in PhoneNumber

ts/types/PhoneNumber.ts

@@ -1,6 +1,7 @@
 import { instance, PhoneNumberFormat } from '../util/libphonenumberInstance';
+import memoizee from 'memoizee';
 
-export function format(
+function _format(
   phoneNumber: string,
   options: {
     ourRegionCode: string;
@@ -21,6 +22,14 @@ export function format(
   }
 }
 
+export const format = memoizee(_format, {
+  primitive: true,
+  // Convert the arguments to a unique string, required for primitive mode.
+  normalizer: function(args) {
+      return JSON.stringify(args);
+  }
+});
+
 export function parse(
   phoneNumber: string,
   options: {