Missing login screen when deep linking into Calypso

[jessefriedman]

Steps to reproduce

1. Starting as a logged out WordPress.com user (incognito window is fine)
2. Go to URL: https://wordpress.com/themes/[any url] example: https://wordpress.com/themes/jesse.blog
3. You should see a nearly blank screen instead of the nicer login screen you'd see https://wordpress.com/stats/jesse.blog

What I expected

I expected to see a screen like this:

What happened instead

I got a screen that is essentially blank:

Browser / OS version

All browsers, all devices/comps

Screenshot / Video

see above

Context / Source

JITMs, hosts, affiliates all deep link to Calypso. Users will also likely bookmark Calypso pages, not to mention newsletters, and other emails that promote Calypso usage. Most of these links are going to blank login screens. We need to present the same login screen consistently no matter where the user is trying to deep link to.

cc/ @johnHackworth @joanrho

[jsnajdr]

Duplicate of #23785.
Has a fix PR in review: #30537.