topic [nfc]: Fix internal references to "subject"

The bulk of the places we mention "subject" are direct references to
either the property of that name on `Message` or somewhere else in
the server API.  Most of those, and in particular `Message#subject`,
are still that way in the current API.

Those will be an annoying giant conversion, so I have no desire to be
in a rush to do them.  In particular, for any given piece of API, I
won't want to make a conversion before we've at least settled on the
API change so that we can convert to what we know is the future API.

However, there are a handful of places where we say "subject" to
mean "topic" and it's entirely internal to the mobile app.  So
here we go ahead and fix those.

Author: gnprice

src/chat/ChatScreen.js

@@ -152,17 +152,15 @@ export default function ChatScreen(props: Props): Node {
     (message: string, destinationNarrow: Narrow) => {
       if (editMessage) {
         const content = editMessage.content !== message ? message : undefined;
-        const subject = caseNarrowDefault(
+        const topic = caseNarrowDefault(
           destinationNarrow,
+          // eslint-disable-next-line no-shadow
           { topic: (streamId, topic) => (topic !== editMessage.topic ? topic : undefined) },
           () => undefined,
         );
 
-        if (
-          (content !== undefined && content !== '')
-          || (subject !== undefined && subject !== '')
-        ) {
-          api.updateMessage(auth, { content, subject }, editMessage.id).catch(error => {
+        if ((content !== undefined && content !== '') || (topic !== undefined && topic !== '')) {
+          api.updateMessage(auth, { content, subject: topic }, editMessage.id).catch(error => {
             showErrorAlert(_('Failed to edit message'), error.message);
           });
         }

src/message/__tests__/messagesReducer-test.js

@@ -321,7 +321,7 @@ describe('messagesReducer', () => {
     test('when event contains a new subject but no new content only subject is updated', () => {
       const message1Old = eg.streamMessage({
         content: 'Old content',
-        subject: 'Old subject',
+        subject: 'Old topic',
         last_edit_timestamp: 123,
         edit_history: [],
       });
@@ -344,11 +344,11 @@ describe('messagesReducer', () => {
     test('when event contains a new subject and a new content, update both', () => {
       const message1Old = eg.streamMessage({
         content: '<p>Old content</p>',
-        subject: 'Old subject',
+        subject: 'Old topic',
         last_edit_timestamp: 123,
         edit_history: [
           {
-            prev_subject: 'Old subject',
+            prev_subject: 'Old topic',
             timestamp: 123,
             user_id: eg.otherUser.user_id,
           },

src/webview/html/header.js

@@ -20,7 +20,7 @@ import {
 } from '../../utils/recipient';
 import { base64Utf8Encode } from '../../utils/encoding';
 
-const renderSubject = message =>
+const renderTopic = message =>
   // TODO: pin down if '' happens, and what its proper semantics are.
   message.match_subject !== undefined && message.match_subject !== ''
     ? message.match_subject
@@ -40,7 +40,7 @@ export default (
   if (message.type === 'stream') {
     const streamName = streamNameOfStreamMessage(message);
     const topicNarrowStr = keyFromNarrow(topicNarrow(message.stream_id, message.subject));
-    const topicHtml = renderSubject(message);
+    const topicHtml = renderTopic(message);
 
     if (headerStyle === 'topic+date') {
       return template`\