This PR adds support for loading media from private Atomic sites.

Companion Flux-C PR needs to be merged first.

There are two main changes:

• If the site is private atomic, we load Photon media using different base URL (media proxy). Most of the changes are related to this method.
• We use a cookie to load media in Editor, Reader post details and site/post preview.

Since the use case for this is relatively small, I wanted to avoid adding too much new logic, and instead integrated the behavior into existing flow with (hopefully) minimal amount of interruption.

There are a couple of steps to test the changes:

• Using a8c account and wpcalypso.wordpress.com (important) create a site with a business plan, perform AT by installing a plugin and set the site to private.
• Using App, switch to the abovementioned site.

Site preview:

• From MySite screen, preview the site on the web by tapping on the site name, or View Site button. You should be able to view the site.

Editor:

• Create a new post, set a featured image to it and add an image to the post content. Publish the post.
• View post from post list. It should be displayed correctly.
• Open post in Block editor. The image in the post should be displayed correctly.
• Open post in Aztec. The image in the post should be displayed correctly.

Reader:

• Navigate to the reader, and select Following tab.
• From subfilter select the private AT site we are testing.
• Notice that the featured image in the post is displayed correctly in the reader post list.
• Open the post details. Notice that the image in the post is displayed correctly.

When testing cookie authentication, you might encounter cookie loading with a modal dialog.
Cookie loading injects into normal content loading flow and interrupts it if necessary. Loading will continue as normal after the cookie is retrieved. A cookie Progress dialog is cancellable, and when canceled the normal flow of content render will resume.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.