markdown: Move code related to @mentions to markdown.js.

For consistency, we should keep all the code that works with
@mentions in markdown.js. In this case, message_list_view was
rewriting the contents of the mentions in cases where users'
names had been changed since we rendered their mention.

frontend_tests/node_tests/markdown.js

@@ -542,3 +542,12 @@ run_test('katex_throws_unexpected_exceptions', () => {
     assert(blueslip.get_test_logs('error').length, 1);
     blueslip.clear_test_data();
 });
+
+run_test('misc_helpers', () => {
+    const elem = $('.user-mention');
+    markdown.set_name_in_mention_element(elem, 'Aaron');
+    assert.equal(elem.text(), '@Aaron');
+    elem.addClass('silent');
+    markdown.set_name_in_mention_element(elem, 'Aaron, but silent');
+    assert.equal(elem.text(), 'Aaron, but silent');
+});

static/js/markdown.js

@@ -36,6 +36,15 @@ var backend_only_markdown_re = [
     /[^\s]*(?:twitter|youtube).com\/[^\s]*/,
 ];
 
+// Helper function to update a mentioned user's name.
+exports.set_name_in_mention_element = function (element, name) {
+    if ($(element).hasClass('silent')) {
+        $(element).text(name);
+    } else {
+        $(element).text("@" + name);
+    }
+};
+
 exports.contains_backend_only_syntax = function (content) {
     // Try to guess whether or not a message has bugdown in it
     // If it doesn't, we can immediately render it client-side

static/js/message_list_view.js

@@ -525,11 +525,7 @@ MessageListView.prototype = {
                 if (person !== undefined) {
                     // Note that person might be undefined in some
                     // unpleasant corner cases involving data import.
-                    if ($(this).hasClass('silent')) {
-                        $(this).text(person.full_name);
-                    } else {
-                        $(this).text("@" + person.full_name);
-                    }
+                    markdown.set_name_in_mention_element(this, person.full_name);
                 }
             }
         });