ui: Rename stream to channel in user-facing strings

Fixes part of #630

Author: Khader-1

lib/widgets/app.dart

@@ -282,7 +282,7 @@ class HomePage extends StatelessWidget {
           ElevatedButton(
             onPressed: () => Navigator.push(context,
               SubscriptionListPage.buildRoute(context: context)),
-            child: const Text("Subscribed streams")),
+            child: const Text("Subscribed channels")),
           const SizedBox(height: 16),
           ElevatedButton(
             onPressed: () => Navigator.push(context,

lib/widgets/message_list.dart

@@ -148,13 +148,13 @@ class MessageListAppBarTitle extends StatelessWidget {
       case StreamNarrow(:var streamId):
         final store = PerAccountStoreWidget.of(context);
         final stream = store.streams[streamId];
-        final streamName = stream?.name ?? '(unknown stream)';
+        final streamName = stream?.name ?? '(unknown channel)';
         return _buildStreamRow(stream, streamName);
 
       case TopicNarrow(:var streamId, :var topic):
         final store = PerAccountStoreWidget.of(context);
         final stream = store.streams[streamId];
-        final streamName = stream?.name ?? '(unknown stream)';
+        final streamName = stream?.name ?? '(unknown channel)';
         return _buildStreamRow(stream, "$streamName > $topic");
 
       case DmNarrow(:var otherRecipientIds):

lib/widgets/subscription_list.dart

@@ -81,7 +81,7 @@ class _SubscriptionListPageState extends State<SubscriptionListPage> with PerAcc
     unpinned.sortBy((subscription) => subscription.name.toLowerCase());
 
     return Scaffold(
-      appBar: AppBar(title: const Text("Streams")),
+      appBar: AppBar(title: const Text("Channels")),
       body: SafeArea(
         // Don't pad the bottom here; we want the list content to do that.
         bottom: false,
@@ -115,7 +115,7 @@ class _NoSubscriptionsItem extends StatelessWidget {
     return SliverToBoxAdapter(
       child: Padding(
         padding: const EdgeInsets.all(10),
-        child: Text("No streams found",
+        child: Text("No channels found",
           textAlign: TextAlign.center,
           style: TextStyle(
             // TODO(#95) need dark-theme color