Adds http_config.response_headers to the UI headers plus tests

[johncowen]

This PR uses the already existing http_config.response_headers user config for setting HTTP headers when serving the UI. Use case here is for setting headers like X-Frame-Options for the UI.

Fixes #5712

Couple of notes:

1. Do we want to reuse this config setting, or would a new one fit better? My thoughts here are it ok to reuse.
2. Codewise I started out here with an anonymous function after the check for whether the UI is enabled, meaning the serveHandlerWithHandlers wrapper function is only created if we are running the UI. I'm not sure how important these things are in the codebase, and in the end I hoisted this function out so it lives with the rest of the functions in this file.

Submitting in Draft for the moment as I'd like to get some feedback on process for working on the go codebase here before I submit proper.

[hanshasselberg]

👍