Using the new configuration variable 'keystore_enable_groups'.

Likely fixes issue #7 (untested).

Without this field, U2F just isn't working and the accounts with U2F
set up are shown an OTP prompt.

There seems to be no good reason to change group membership when
exchanging tokens, let's try just passing the old one over.

Using CORS-enabled requests in the background.

Allow loading remote images.