The OTP server variables were read straight from the environment in
views.py. They are now OTP_* settings next to everything else, under
the same environment variable names, so existing deployments are
unaffected.
Remove LDAP_SERVER and LDAP_USERS_DN, which nothing used.
env.sample moves to the repository root (where .env is read from) and
lists every variable settings.py reads, including defaults and which
values are secrets.
