Allow getTitanSetUpMailboxPath for domain only sites

[lsl]

Fixes #95147

Proposed Changes

• Allows getTitanSetUpMailboxPath access for domain only sites

Why are these changes being made?

• When you remove all your mailboxes you're left with the option to "set up mailbox" for the unused slot.
• Clicking on this takes you to http://calypso.localhost:3000/email/testdomainonlysite123.blog/titan/set-up-mailbox/testdomainonlysite123.blog
• Domain only sites previously restricted this path resulting in a redirect to domains/manage
• While /mailboxes/:siteslug route already has access, when no mailboxes are found it would redirect to this location as well.

Testing Instructions

• From /domains/manage (Manage all domains page), purchase a domain without it being attached to a site.
• From /email/:domain/manage/:domain (Manage emails page), purchase a titan mail email box
• Delete the mailbox
• Attempt to click on "Mailboxes" in the sidebar, it should re-route you to mailbox setup.
• Click "Manage Email" in the sidebar, click the "Set up mailbox" cta in blue, it should take you to the setup page
• Click "Manage Email" again, click "Add a mailbox", it should show a warning and provide a link to setup mailbox again, click that, the setup page should be shown again.

These links should no longer redirect to /domains/manage

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-2f1bce4b8fa1b98214b4bf952fc30ccd47635767

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-2f1bce4b8fa1b98214b4bf952fc30ccd47635767&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-2f1bce4b8fa1b98214b4bf952fc30ccd47635767&env=a8c-for-agencies

[matticbot]

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

App Entrypoints (~2 bytes added 📈 [gzipped])

name           parsed_size           gzip_size
entry-stepper        +26 B  (+0.0%)       +2 B  (+0.0%)
entry-main           +26 B  (+0.0%)       +1 B  (+0.0%)
entry-login          +26 B  (+0.0%)       +1 B  (+0.0%)

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

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.

[fushar]

It works! Although as a user I'm confused why we have two terms "Email" and "Mailbox" in the sidebar which does the same thing (but of course it's out of the scope of this PR).