Invalidate `http_response` for new scripts or change in placement

Problem/Motivation

When using `page_cache` or a reverse proxy like Varnish, the `http_response` cache tag needs to be invalidated when:

- A new script is created
- Script placement is changed

Otherwise cached pages will be missing the snippet.

Steps to reproduce

I haven't manually tested yet, but I assume:

- Visit `/` anonymous
- Add a script
- Revisit `/` and no script

Proposed resolution

Whenever a script is saved, invalidate `http_response` cache tag

Remaining tasks

User interface changes

API changes

Data model changes