Move email templates to data

[evankanderson]

Summary

The current email template contents are both hard-coded and Stacklok-specific. Move these to kodata, so that other users can customize their own email templates.

I also noticed that we were using NewSafeHTMLTemplate for the text body, and re-parsing the templates every time -- this is now moved to module-level initialization (this means you need to restart the server to pick up new templates, but this seems reasonable given that other config requires a restart

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Unit tests (so far) -- will do some integration testing after merge.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.