api: Check for post_max_size on api requests
php documentation:
If the size of post data is greater than post_max_size, the $_POST and
$_FILES superglobals are empty.
When the action= and format= are not in the GET data,
the help page is returned in html, breaking the clients expected format.
Return api error with http status 413
Bug: T291754
Change-Id: I5906fb6b4412b161b198df0b51e2476e7e1079b8