Defer Banzai context member resolution until actually used by filters

What does this MR do and why?

CacheMarkdownField#banzai_render_context is called for every single Markdown field render in the site. This includes when the rendered HTML is cached in the database row for the field: we still need the render context, because every render must pass through the post-processing pipeline.

At the moment, this method always calls self.project, self.group if self responds to :group, same for self.author (notes in particular use this), and self.parent_user (only non-nil for snippets). For non-cached renders, these values are almost always all going to be used by the main pipeline they're rendered with. For cached renders, however — the majority of all renders — only the post-processing pipeline runs, and in many instances, the group, author and parent_user will never be consulted.

This is probably the hottest path that exists in the Markdown pipeline, and we might be able to make a noticeable dent on database read traffic by removing unnecessary lookups. This MR seeks to measure this dent by making these lookups lazy, conditional on a FF. If it's noticeable, it's worth!, and we should roll out the FF. If not, let's not complicate things for no gain.

References

See CacheMarkdownField.banzai_render_context forces... (#589496 - closed).

How to set up and validate locally

There is no user-visible change, so instead we seek to observe the difference before-and-after on queries made when rendering from cache.

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