Initial H5P support

This issue should be tested both in Android and iOS. Test packages hosted in h5p.org In Moodle 3.8/master as admin/teacher, create a new label or page activity. In the Atto editor, click the H5P button and add a URL to an h5p package in h5p.org (there are some in the testing instructions of  MDL-66397 ). Please notice there's a page already created in Master site > MM Dev Testing > H5P > Page with external H5P. In the app as student, open the page/label. Check that you see a "placeholder" with the H5P logo and the play button, but no download button. Click Play. Check that you can use the H5P (in online). Test packages hosted in the site Add any of the packages attached to this issue to a Moodle site, in labels. There are some already added in Master site > MM Dev Testing > H5P, in different labels. The reason to add them in labels is that pages are automatically downloaded when opened in the app. In the app as student, go to the H5P. Check that you see a "placeholder" with the H5P logo, a play button and a download button. Click Play. Check that you can use the H5P almost immediately (you don't have to wait for it to be downloaded and extracted, this could take a few seconds). Perform a PTR. Check that you see the placeholder again, but now the download button displays the "downloaded" icon (the package was downloaded in background). Go offline and click Play. Check that you can play the package. Now go to App settings > Space usage and delete all the files from the site. Go online again, go back to the H5P package and click the download button. Check that the package is downloaded successfully. Go offline and click Play. Check that you can play the package.