Add an authentication layer

[matejak]

In the longer run, the TEAP app should have a first-class citizen option to enable authentication, and subsequent authorization.
The enterprisey solution is a SAML2 option with keycloak with LDAP on the server side. Ideally, the authentication should allow a Google sign-in option as well (e.g. based on flask-dance), so the SAML2 solution should be implemented in such way that adding additional IDM backends shouldn't require heavy rewrites.