Load editor dynamically

Don't load it on every page load, only when the edit icon is clicked.
Non-existent page data transferred (stable, desktop Chrome)
before: 328KB, after: 319KB

Reuse LoadingOverlay from Nearby previews to show a spinner when loading
JS dynamically. Remove the message from the LoadingOverlay (we use
spinners without messages everywhere to indicate that something is
loading, it's a standard pattern that everyone understands).

Bug: 53782
Tracking bug: 48718
Change-Id: I315f8d0404a7b2edaa0579dd747f9143aa5d3c38
Merge "Load editor dynamically"