Limit new messages length

lib/ui/screens/conversation_screen.dart

@@ -244,14 +244,29 @@ class _ConversationScreenState extends State<ConversationScreen> {
           // Message area
           new Flexible(
             child: new TextField(
+              maxLines: null,
+              maxLength: 200,
+              maxLengthEnforced: true,
+
+              // Show max length only when there is some text already typed
+              buildCounter: (
+                BuildContext context, {
+                @required int currentLength,
+                @required int maxLength,
+                @required bool isFocused,
+              }) =>
+                  currentLength > 0
+                      ? Text("$currentLength/$maxLength")
+                      : Container(),
               enabled: !_isSendingMessage,
               controller: _textEditingController,
               onChanged: _updatedText,
               onSubmitted: _isMessageValid
                   ? (s) => _submitTextMessage(context, s)
                   : null,
-              decoration:
-                  new InputDecoration.collapsed(hintText: tr("Send a message")),
+              decoration: new InputDecoration.collapsed(
+                hintText: tr("Send a message"),
+              ),
             ),
           ),