Fix CORS issue on Firefox when attempting to join classroom

[GuillermoFloresV]

Describe the bug
The bug has been temporarily patched with this commit: 40e7ab1

However, this is a temp fix as we should look into why there is a CORS issue when using this API on Firefox. The issue does not appear on Chrome and has not been tested on Microsoft Edge.

To Reproduce
Steps to reproduce the behavior:

1. Go to '/pages/join/[...joincode].js'
2. Remove/comment out event.preventDefault()
3. Attempt to invite someone to the classroom
4. See error (TypeError: NetworkError when attempting to fetch resource)

Additional context

The bug is currently patched, but we should look into the underlying cause to fix it without bypassing CORS the way we did.