Fix CORS issue on Firefox when attempting to join classroom

**Describe the bug**
The bug has been temporarily patched with this commit: https://github.com/freeCodeCamp/classroom/commit/40e7ab1f1ea75109c50c23988cc60a02e8d20c56

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.