Update security template

Why is this change being made?

This was the single template that was the odd one out as it isn't part of the handbook, but expects to use it's toc related handbook.js code and CSS. I've updated this follow-up to additionally consider refactoring the JS use into smaller units.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies

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Closes #6853 (closed)