Added "Switch user" functionality.

The user isn't logged out until the new user logs in. This allows you to
click on "Log in as different user", hit the back button, and still be
logged in.

pillar/api/blender_id.py

@@ -284,3 +284,12 @@ def fetch_blenderid_user() -> dict:
 
 def setup_app(app, url_prefix):
     app.register_api_blueprint(blender_id, url_prefix=url_prefix)
+
+
+def switch_user_url(next_url: str) -> str:
+    from urllib.parse import quote
+
+    base_url = '%s/switch' % blender_id_endpoint()
+    if next_url:
+        return '%s?next=%s' % (base_url, quote(next_url))
+    return base_url

pillar/web/users/routes.py

@@ -107,6 +107,23 @@ def logout():
     return redirect('/')
 
 
+@blueprint.route('/switch')
+def switch():
+    from pillar.api import blender_id
+
+    # Without this URL, the Cloud will redirect to the HTTP Referrer, which is the Blender ID
+    # 'switch user' page. We need to explicitly send the user to the homepage to prevent this.
+    next_url_after_cloud_login = url_for('main.homepage')
+
+    # Without this URL, the user will remain on the Blender ID site. We want them to come
+    # back to the Cloud after switching users.
+    next_url_after_bid_login = url_for('users.login',
+                                       next=next_url_after_cloud_login,
+                                       _external=True)
+
+    return redirect(blender_id.switch_user_url(next_url=next_url_after_bid_login))
+
+
 @blueprint.route('/settings/profile', methods=['GET', 'POST'])
 @login_required
 def settings_profile():