Use just the timestamp for story expiration

ts/models/messages.ts

@@ -3042,13 +3042,7 @@ export class MessageModel extends window.Backbone.Model<MessageAttributesType> {
       }
 
       if (!message.get('expirationStartTimestamp')) {
-        message.set(
-          'expirationStartTimestamp',
-          Math.min(
-            message.get('serverTimestamp') || message.get('timestamp'),
-            Date.now()
-          )
-        );
+        message.set('expirationStartTimestamp', message.get('timestamp'));
         changed = true;
       }
     }

ts/services/storyLoader.ts

@@ -80,10 +80,7 @@ async function repairUnexpiredStories(): Promise<void> {
     )
     .map(story => ({
       ...story,
-      expirationStartTimestamp: Math.min(
-        story.serverTimestamp || story.timestamp,
-        Date.now()
-      ),
+      expirationStartTimestamp: Math.min(story.timestamp, Date.now()),
       expireTimer: Math.min(
         Math.floor((story.timestamp + durations.DAY - Date.now()) / 1000),
         DAY_AS_SECONDS

ts/textsecure/MessageReceiver.ts

@@ -1949,9 +1949,7 @@ export default class MessageReceiver
     }
 
     const expireTimer = Math.min(
-      Math.floor(
-        (envelope.serverTimestamp + durations.DAY - Date.now()) / 1000
-      ),
+      Math.floor((envelope.timestamp + durations.DAY - Date.now()) / 1000),
       durations.DAY / 1000
     );
 
@@ -1964,7 +1962,7 @@ export default class MessageReceiver
       return;
     }
 
-    const message = {
+    const message: ProcessedDataMessage = {
       attachments,
       canReplyToStory: Boolean(msg.allowsReplies),
       expireTimer,