Enter UUID in sign up form

[mzur]

Add a checkbox to the sign up form "I already have an account in another instance". If the checkbox is checked, add a required field "User UUID" in which the user UUID from the other instance should be entered. Add a "?" button to the field with the help text: "You can find your user UUID in your user account settings of the other BIIGLE instance."

[mzur]

Alternative: Offer a public user export where a user can export their own data as JSON file. In the sign up form of another BIIGLE instance, the user can upload the JSON file to pre-fill the form fields (including the hidden UUID field). They only have to enter the password again.