Backport of "Fix undefined method markdown_placeholders_feature_flag_enabled? for a ProjectNamespace"

What does this MR do and why?

This is a backport of Fix undefined method `markdown_placeholders_fea... (!201333 - merged)

The log https://log.gprd.gitlab.net/app/discover#/?_g=h@e5dc602&_a=h@d054e3c shows

    "class": "NoMethodError",
        "message": "undefined method `markdown_placeholders_feature_flag_enabled?' for #<Namespaces::ProjectNamespace id:34990267 @hindman/infra/support>"

in lib/banzai/filter/markdown_engines/glfm_markdown.rb:88:in 'placeholders_disabled?'. This should have already been protected by !198821 (merged), but this still seems to be occurring.

There is no workaround, and several customers, including self-managed, have asked for a fix in 18.2

Related to #556207 (closed)

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
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• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.