PR Summary

Added a small change to the JavaScript for the WebAgg backend. The added JavaScript code will check the protocol and updated the websocket uri to wss: if https: is being used.

This is to fix an issue with matplotlib running behind a reverse proxy that adds HTTPS and secure HTTP headers like HSTS. The connection to the insecure websocket i.e. ws: is blocked by the browser with the message:

Mixed Content: The page at 'https:/<some_website>/' was loaded over HTTPS,  but attempted to connect to the insecure 
WebSocket endpoint 'ws://<some_website>/1/ws'. This request has been blocked; this endpoint must be available over WSS.

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes) N/A
• Documentation is sphinx and numpydoc compliant (the docs should build without error). N/A
• New plotting related features are documented with examples. N/A

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/ N/A
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/ N/A
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst N/A

Not sure whether or not I need to update any documentation, so please let me know if I do.

Thank you