Resolve "Custom renderers cleanup - erb"

What does this MR do?

It iterates on the previously introduced templater static site editor pre-processing step to handle two additional use-cases:

• Content that contains inline ERB
• HTML blocks
  • If this templater solution proves robust enough, we could potentially remove the render_html_block.js custom renderer. It was currently left in as HTML comment syntax isn't accounted for in this MR.

We'll need at least one more follow-up to handle existing code blocks (we want to leave those as-is)

Screenshots

before	after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team