PLT-2809 Followups to login page redesign (#2901)

* Rearranged controls on the login page

* Prevented login when username or password hasn't been entered

* Updated login page error messages

* Enabled sign in with username by default

config/config.json

@@ -78,7 +78,7 @@
     "EmailSettings": {
         "EnableSignUpWithEmail": true,
         "EnableSignInWithEmail": true,
-        "EnableSignInWithUsername": false,
+        "EnableSignInWithUsername": true,
         "SendEmailNotifications": false,
         "RequireEmailVerification": false,
         "FeedbackName": "",