Implement support for authentication via Auth Proxy

Auth Proxy allows to authenticate a user using an HTTP header provided
by an external authentication service. This provides a way to
authenticate users in miniflux using authentication schemes not
supported by miniflux itself (LDAP, non-Google OAuth2 providers, etc.)
and to implement SSO for multiple applications behind single
authentication service.

Auth Proxy header is checked for the '/' endpoint only, as the rest are
protected by the miniflux user/app sessions.

Closes #534

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>