Replace static JavaScript fixtures for u2f

What does this MR do?

Generates HAML fixtures for frontend u2f tests dynamically.

Are there points in the code the reviewer needs to double check?

All builds should pass.

Why was this MR needed?

The fixtures used in u2f/authenticate_spec.js and u2f/register_spec.js are generated directly from a partial template and include inline <script> tags which can cause issues in certain test runners. These currently pass within Teaspoon, but only because of a quirk in the timing of the call to fixture.load. This MR changes the way the fixture is rendered, and scrubs out all inline script tags from the generated html.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related to #24753 (closed)