Draft: Add caching for project member invite modal view

What does this MR do and why?

Add caching for project member invite modal view.

This view is rendered on every single project page that uses app/layouts/projects.html.haml if the user has the required permissions, and performs a relatively expensive query (10-15% of the overall page load time) on every page load for a rarely-used feature.

This MR adds a cache around the modal partial to reduce the number of expensive queries. Long-term this modal would make more sense as a separate page rather than adding this load-time onto every page load.

Changelog: performance

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