Marketplace installation: Support externally managed themes

[ryelle]

Related to #95651

Proposed Changes

Update the marketplace theme install to support "externally managed" themes (partner themes).

Why are these changes being made?

We want to use the marketplace install process to install both community and partner themes, since it handles the atomic transfer. Currently it works, more by accident— because it still installs the theme even though it doesn't recognize it as a community theme.

Testing Instructions

• Gather a few community and partner themes to test with
  • Community: Find on https://wordpress.org/themes/, get the slug from the URL
  • Partner: Find on https://wordpress.com/themes/partner, get the slug from the URL
• Manually enter the marketplace install route, e.g., /marketplace/theme/<theme slug>/install/<site>
• You should see the correct thumbnail and theme title, not the theme slug
• The install process should work, assuming you have purchased the partner theme subscription

Before	After

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-133195

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-133195&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-133195&env=a8c-for-agencies

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug update/marketplace-install-partner-themes on your sandbox.

[matticbot]

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

Sections (~15 bytes added 📈 [gzipped])

name         parsed_size           gzip_size
marketplace        +57 B  (+0.0%)      +15 B  (+0.0%)

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.

[ryelle]

I changed this to externalTheme since it's not necessarily a wp.org theme, but "external" might sound too much like "externally managed" aka partner themes, so open to other suggestions.