Only calculate supported post types for author taxonomy once per request

[douglas-johnson]

Description

This is intended to prevent recaculating the supported post types for the author taxonomy. The problems with the recalculation are laid out thoroughly in #1049

This was also solved in #1036 however I'm not able to re-open someone else's pull request. Thanks @justinmaurerdotdev for figuring this out the first time.

Steps to Test

If you add logging before and after the lines changd in this PR you should see 100s of calls to supported_post_types() when loading the editor, but it should only proceed with calculation once per request.

For example:

error_log('getting supported post types');
if ( ! empty( $this->supported_post_types ) ) {
    return $this->supported_post_types;
}
error_log('"$_SERVER['REQUEST_URI']");
error_log('calculating supported post types');

[justinmaurerdotdev]

Oh, hey. Nice!