fetch: Retry fetching messages, indefinitely.

Fixes #3281.

This simple change greatly improves the experience with spotty
network connections.

I have a small concern about the request's retry not being cancelled.
I find the compromise to be worth it.  I am working on a more generic
rework of `tryUntilScuccessful` which will allow it to be cancelled
and will include all of this function's invocations.

src/message/fetchActions.js

@@ -61,13 +61,8 @@ export const fetchMessages = (
   useFirstUnread: boolean = false,
 ) => async (dispatch: Dispatch, getState: GetState) => {
   dispatch(messageFetchStart(narrow, numBefore, numAfter));
-  const { messages, found_newest, found_oldest } = await getMessages(
-    getAuth(getState()),
-    narrow,
-    anchor,
-    numBefore,
-    numAfter,
-    useFirstUnread,
+  const { messages, found_newest, found_oldest } = await tryUntilSuccessful(() =>
+    getMessages(getAuth(getState()), narrow, anchor, numBefore, numAfter, useFirstUnread),
   );
   dispatch(
     messageFetchComplete(messages, narrow, anchor, numBefore, numAfter, found_newest, found_oldest),