compose_ui: Automatically resize textarea after adding new syntax.

This fixes existing problems as well as an upcoming problem that
copy-and-pasting in markdown should usually expand the compose text
area.

frontend_tests/node_tests/compose_ui.js

@@ -38,6 +38,10 @@ function make_textbox(s) {
         return widget.s;
     };
 
+    widget.trigger = function () {
+        return;
+    };
+
     return widget;
 }
 

static/js/compose_ui.js

@@ -29,6 +29,9 @@ exports.smart_insert = function (textarea, syntax) {
 
     textarea.caret(syntax);
     textarea.focus();
+    // This should just call exports.autosize_textarea, but it's a bit
+    // annoying for the unit tests, so we don't do that.
+    textarea.trigger("autosize.resize");
 };
 
 exports.insert_syntax_and_focus = function (syntax) {