Login into Joplin staging give a 403 error page

## Describe the bug
A clear and concise description of what the bug is
When I try to log into Joplin Staging
## URL
https://joplin-staging.herokuapp.com/
## To Reproduce
Steps to reproduce the behavior
example:
1. Go to 'https://joplin-staging.herokuapp.com/'
2. Try to log in
3. User is redirected to the error screen (see the attachment)
4. See error

The user is not logged into the page unless he/she/they clicks the back button in the browser

## Current behavior
A clear and concise description of what happened
User is directed to a error page (403 see attachment)
## Expected behavior
A clear and concise description of what you expected to happen
user should be logged into the page after the right credential are entered in the login page.
## Screenshots

## Device and browser

## Additional context