Added basic LDAP signin without tests. Manually tested against OpenLDAP and Active Directory with plain authentication.
Show first LDAP signin page if it's the only provider.
Add /favicon.ico to unauthenticated routes, so it doesn't end up as the referrer when we log in with LDAP.
Added LDAP documentation, cleaned up get_signin_redirect().
Merge remote-tracking branch 'ldap/ldap_signin'

Conflicts:
	Gemfile
	Gemfile.lock