Got usernames working, for now, I think.

apps/bwc_web/assets/js/bwc.js

@@ -7,7 +7,7 @@ window.UIkit = UIkit;
 window.show_login_box = () => {
 
     $("#top-bar-username a").hide();
-    $("#top-bar-login-box").show().focus();
+    $("#top-bar-login-box").val($("#top-bar-username a").text().trim()).show().focus();
 
 }
 
@@ -37,7 +37,7 @@ $("#top-bar-login-box").keyup(function(e) {
 
 window.login = () => {
     var CSRF_TOKEN = $("input[name=_csrf_token]").val();
-    var username = $("#top-bar-login-box").val();
+    var username = $("#top-bar-login-box").val().trim();
     $.ajax({
         url: "/api/join",
         method: 'POST',
@@ -45,13 +45,14 @@ window.login = () => {
             xhr.setRequestHeader("X-CSRF-Token", CSRF_TOKEN);
         },
         data: {
-            username: username
+            username: username,
+            old_username: $("#top-bar-username a").text().trim()
         }
     }).done(function(response) {
         console.log(response);
         UIkit.notification(response['data']);
         hide_login_box();        
-        $("#top-bar-username a").text(response['username']);
+        $("#top-bar-username a").text(response['username']);        
     });
 }