Photon: do not apply Photon when doing an AJAX request in the admin

[jeherve]

We already exclude Photon from being applied on admin, but not when working with AJAX in the admin.

Reported here:

• ​http://wordpress.org/support/topic/jetpack-photon-with-admin-ajaxphp?replies=1

[jeherve]

Photon: do not apply Photon when doing an AJAX request in the admin

[migueluy]

In 803836:

Error: Failed to load processor CommitTicketReference

No macro or processor named 'CommitTicketReference' found

[jeherve]

In r806705:

Revert r803836 - Photon should not be applied when inserting images into a post via AJAX
Reopens #1885

[jeherve]

Moved to GitHub:

• ​https://github.com/Automattic/jetpack/issues/722