Replace U2F with WebAuthn (#1392)

* Replace U2F with WebAuthn

* Imports

* Fix backwards compatibility

* Add explanatory comment

* Fix tests

src/pretix/helpers/webauthn.py

@@ -0,0 +1,24 @@
+import random
+import string
+
+
+def generate_challenge(challenge_len):
+    return ''.join([
+        random.SystemRandom().choice(string.ascii_letters + string.digits)
+        for i in range(challenge_len)
+    ])
+
+
+def generate_ukey():
+    """
+    Its value's id member is required, and contains an identifier
+    for the account, specified by the Relying Party. This is not meant
+    to be displayed to the user, but is used by the Relying Party to
+    control the number of credentials - an authenticator will never
+    contain more than one credential for a given Relying Party under
+    the same id.
+    A unique identifier for the entity. For a relying party entity,
+    sets the RP ID. For a user account entity, this will be an
+    arbitrary string specified by the relying party.
+    """
+    return generate_challenge(20)