Cache the dialog boxes

[zanematthew]

The loading of the dialog boxes should be cached and not a separate AJAX request each time.

[zanematthew]

https://wordpress.org/support/topic/problem-with-translation-12?replies=3#post-6492797

[zanematthew]

AJAX request (when using POST) are not cached by default, see the AJAX docs and reference the section on "cache".

What does happen is that the forms are reloaded every time the user clicks login/register. Rather than have the AJAX request load new forms each time, we can pre-load them once the page loads.