When logging in from /welcome, redirect to index

Backend logic simplified, to support the next arg if provided, otherwise simply redirect to the request.referrer (last visited page).

cloud/routes.py

@@ -132,13 +132,8 @@ def login():
 
     next_after_login = request.args.get('next')
 
-    # Redirect to /welcome if explicitly given, but not when falling back to the referrer.
     if not next_after_login:
-        url_for_welcome = url_for('cloud.welcome', _external=True)
-        if request.referrer == url_for_welcome:
-            next_after_login = '/'
-        else:
-            next_after_login = request.referrer
+        next_after_login = request.referrer
 
     session['next_after_login'] = next_after_login
     return redirect(url_for('users.oauth_authorize', provider='blender-id'))