Add cloak-startup class and apply to mobile overlay

What does this MR do and why?

We currently have several specific elements cloaked to prevent flashing when using startup.css. This MR introduces a generic class which can be applied to any element to achieve the same.

Here we apply it to the mobile top nav overlay to fix broken rendering in certain race/network conditions.

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Block application.css from loading in your browser (dev tools/network tab)
2. Hit any page

If you do this with master checked out, you should see the "before" screenshot from above (the odd looking navigation items in the body)

If you do this with this branch checked out, you should see the "after" screenshot with the blank body

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.