Reference H5P resizer script for activities embedded from external sources

[mcgratay]

Prerequisites

• [x] Can you reproduce the behavior you wish to address with all other plugins deactivated?
• [x] Are you running the latest version of Pressbooks and the latest version of WordPress?
• [x] Are you experiencing this issue with one of our built-in root or book themes?
• [x] Did you perform a cursory search to see if your bug or enhancement is already listed?

Description

Videos from YouTube and Vimeo resize automatically, but video iframes hotlinked from H5P do not. Add H5P domain

Example iframe: <iframe src="https://cdu.h5p.com/content/1290475711255620999/embed" width="560" height="380" frameborder="0"></iframe>

Domain: h5p.com

Example test book: https://h5presize.textopress.com/chapter/chapter-1/

Note: Interactive videos hosted in a book's H5P library are responsive - this problem only affects hotlinked iframes from external H5P sources.

Steps to Reproduce

1. Find a H5P interactive video activity and copy the iframe embed code
2. Open a new chapter and paste the iframe code into the text editor
3. Save the chapter and view in webbook format

Expected behavior: Video resizes with screen

Actual behavior: Video size is static

[SteelWagstaff]

Will look at in more detail, but we should note that H5P activities already have their own resizer script: https://github.com/h5p/h5p-php-library/blob/master/js/h5p-resizer.js and https://h5p.org/comment/23140#comment-23140. When an H5P activity is embedded from elsewhere, it attempts to include a reference to this resizer, but we strip it out for security reasons. See conversation here: https://github.com/hypothesis/client/issues/509#issuecomment-320594957 for more context/options.