Improve GLFM static HTML snapshots

What does this MR do and why?

Related: Tracking Issue: Implement GLFM scripts per the ... (#361241 - closed)

NOTE: Please do not squash commits when merging

Improve GLFM static HTML snapshots.

• Generate via Banzai instead of helper. Renders newlines in a way that matches API results.
• Also creates stub user and project for use in context

See https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide/#update-example-snapshotsrb-script for more context on this and related scripts.

How to set up and validate locally

• Execute scripts/glfm/update-example-snapshots.rb
• It should not fail
• There should be no encoded newlines left in spec/fixtures/glfm/example_snapshots/html.yml (\n in preformatted HTML is expected, and there may be other changes, this is still a work in progress).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.