Uploading a H5P activity into an existing activity

[mattgig]

When you have an existing H5P activity and try to upload another H5P activity into it, all you get is a spinning wheel.

See a more detailed description of the problem in the Moodle forum: https://moodle.org/mod/forum/discuss.php?d=436198#p1755392

[otacke]

@mattgig Cmp. https://github.com/h5p/h5p-editor-php-library/pull/144