Render label notes lazily

What does this MR do and why?

This bypasses the cached Markdown in resource_label_events.reference_html so that we can drop the column.

That column is taking up majority of the disk space for this table (128 GB out of 158 GB).

References

Related to #521854

How to set up and validate locally

1. Enable the render_label_notes_lazily feature flag.
2. Visit issue / MR / epic pages and change some labels
3. Validate that the label system notes are rendered the same whether the FF is enabled / disabled

MR acceptance checklist

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