Fix off by one error

web/react/utils/text_formatting.jsx

@@ -160,11 +160,11 @@ function autolinkAtMentions(text, tokens) {
 
         for (let c = usernameLower.length; c > 0; c--) {
             if (isPunctuation(usernameLower[c - 1])) {
-                usernameLower = usernameLower.substring(0, c);
+                usernameLower = usernameLower.substring(0, c - 1);
 
                 if (mentionExists(usernameLower)) {
-                    const extraText = originalUsername.substr(c);
+                    const extraText = originalUsername.substr(c - 1);
-                    const alias = addToken(usernameLower, mention, extraText);
+                    const alias = addToken(usernameLower, '@' + usernameLower, extraText);
                     return prefix + alias;
                 }
             } else {