livebook: Unable to use iframes (VegaLite) behind HTTPS proxy

Environment

• Elixir & Erlang/OTP versions (elixir --version): Elixir 1.13.2 / Erlang/OTP 24
• Operating system: x86_64-pc-linux-gnu
• How have you started Livebook (mix phx.server, livebook CLI, Docker, etc): docker-compose build
• Livebook version (use git rev-parse HEAD if running with mix): 70ccaafa14fc736e8c7c08ee29ba40250ef8f125
• Browsers that reproduce this bug (the more the merrier): Safari
• Include what is logged in the browser console: Refused to load https://livebook.space/iframe/v3.html because it does not appear in the child-src directive of the Content Security Policy.
• Include what is logged to the server console: n/a

Current behavior

Prior to this commit: e5e13d86c25695f14f11e090efe21533066f9288, I was able to run LiveBook behind an HTTPS proxy server and utilize VegaLite (and other web-based libraries).

Expected behavior

I would expect to be able to define an origin source (that is internal to our network) for establishing the iframe in an HTTPS environment (see: https://github.com/livebook-dev/livebook/blob/62140eb8d75dd5345d1e425551a93df11257eeba/assets/js/hooks/js_view/iframe.js#L45).