types: Make sender_id on Outbox required.

As v27.157 has been out for well over a few weeks.

And write a migration to strip any outbox messages that don't have
it, as noted we should in a1fad7c's TODOs and in #3998.

Author: chrisbobbe

src/boot/store.js

@@ -292,6 +292,12 @@ const migrations: {| [string]: (GlobalState) => GlobalState |} = {
     },
   }),
 
+  // Make `sender_id` on `Outbox` required.
+  '29': state => ({
+    ...state,
+    outbox: state.outbox.filter(o => o.sender_id !== undefined),
+  }),
+
   // TIP: When adding a migration, consider just using `dropCache`.
 };
 

src/message/getHtmlPieceDescriptors.js

@@ -34,10 +34,7 @@ export default (
       });
     }
 
-    // TODO(#3764): Use sender_id, not sender_email.  Needs making
-    //   Outbox#sender_id required; which needs a migration to drop Outbox
-    //   values that lack it; which is fine once the release that adds it
-    //   has been out for a few weeks.
+    // TODO(#3764): Use sender_id, not sender_email.
     const shouldGroupWithPrev =
       !diffRecipient
       && !diffDays

src/types.js

@@ -184,11 +184,7 @@ export type Outbox = $ReadOnly<{|
 
   // The remaining fields are modeled on `Message`.
 
-  // TODO(#3764): Make sender_id required.  Needs a migration to drop Outbox
-  //   values that lack it; which is fine once the release that adds it has
-  //   been out for a few weeks.
-  //   (Also drop the hack line about it in MessageLike.)
-  sender_id?: UserId,
+  sender_id: UserId,
 
   /* eslint-disable flowtype/generic-spacing */
   ...SubsetProperties<
@@ -241,7 +237,6 @@ export type MessageLike =
   | $ReadOnly<{|
       // $Shape<T> is unsound, per Flow docs, but $ReadOnly<$Shape<T>> is not
       ...$Shape<{| [$Keys<Message>]: void |}>,
-      sender_id?: UserId, // TODO: Drop this once required in Outbox.
       ...Outbox,
     |}>;