adjust iframe width based on window size

[mrblippy]

Checklist

• [x] the contributor license agreement is signed
• [x] commit message follows commit guidelines
• [x] screenshots are included showing significant UI changes

Description of change

https://github.com/openequella/moodle-mod_openEQUELLA/issues/52

The iframe now uses an 80% width of the window. But currently this change doesn't really do much, due to oEQ selection session css using fixed widths (fixing that would require an additional change on the core oEQ codebase)