"Refund an arbitrary amount" doesn't validate the import

[yankedev]

Describe the bug
When a ticket is bought using stripe there is the possibility to refund the ticket directly in the admin view.

In the amount input box I can enter any text (letters and commas) and there is no error in the GUI
Only after approving the pop-up message, the admin application gets an error 500 from the server.

In the developer tools I tried changing the input type="text" to "number" and the error is solved

To Reproduce
Steps to reproduce the behavior:

1. Go to '/admin#/events/xxx/reservation/xxx'
2. Insert some letters in the field "Amount" inside the "Refund an arbitrary amount" box
3. Click on 'Refund'
4. Click on 'OK' in the modal that appears
5. See error 500 appearing as toast message

Expected behavior
the refund field should only accept numbers and a dot (.)