Full Site Editing: use correct wp-admin url and params

[mattwiebe]

Changes proposed in this Pull Request

• Use the correct wp-admin URL and params for the site editor.

WordPress/gutenberg#38817 introduced a redirect to ensure that homepage editing works properly. Unfortunately this scrambled a lot of our (fragile!) site editor integration code on the backend.

Testing instructions

You need D76621-code on your sandbox. Sandbox a site with FSE active and also public-api.

Try to load the site editor. It should work with both prod and edge gutenberg versions. But, lacking either the backend patch, or this branch, the site editor redirects on edge to the wp-admin URL at the top level.

Related to #61776 #61858

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-29308

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-29308&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

App Entrypoints (~36 bytes removed 📉 [gzipped])

name        parsed_size           gzip_size
entry-main       -108 B  (-0.0%)      -36 B  (-0.0%)

Common code that is always downloaded and parsed every time the app is loaded, no matter which route is used.

Sections (~9443 bytes added 📈 [gzipped])

name              parsed_size           gzip_size
gutenberg-editor     +12017 B  (+2.4%)    +3909 B  (+2.5%)
themes                 -195 B  (-0.0%)     -893 B  (-0.5%)
theme                  -195 B  (-0.0%)    -1064 B  (-0.8%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[mattwiebe]

Note that I think we need to land D76620-code first in order to get that test to pass

edit: it's landed and I force-pushed. Let's find out!

edit x2: 🎉