[IMP] Authenticator refactoring, new config, signin redesign

- Added the flag SSOActive to the configuration of the authenticators.
- Modified the configuration schema for the authenticators.
- Refactor LDAP authenticator.
- Redesign of the SignIn page.
[REF] Moved NICE calls to NiceAuthenticator.

- Also, refactored LDAP classes and functions.
[REF] Adapted LDAP for CERN
[IMP] Implemented Generic Cache match AvatarHolder
[DEL] Deleted NiceAuthentication related
[DEL] More obsolete code deleted
[IMP] Unit tests
[DOC] Documentation to the base Authenticator

- Also deleted some code.
- Closes #939
[FIX] Post rebase fix
[FIX] Small fixes and PEP8
[FIX] LDAPGroup class moved to LDAPAuthentication

- Also migration script to remove NiceIdentities
[FIX] Various fixes after rebasing
[IMP] Nicer SSO URLs
[FIX] Autofetching identity from ldap if no exists
[FIX] Post-rebase fixes related to external auth