[WIP] Add downloadable template security checklist

[craddm]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).

🚦 Depends on

Adds a downloadable blank template security checklist in Markdown format, linked on the Security Checklist page

🌂 Related issues

🔬 Tests

Documentation only

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
data_safe_haven/commands
users.py					167, 172, 277
Project Total

This report was generated by python-coverage-comment-action

[JimMadge]

😱 I've put some similar changes in on the release branch.

Happy to merge these if there are any differences.