Support LDAP login (LDAPAuthenticator)

[manics]

Closes #264

Based on https://github.com/jupyterhub/ldapauthenticator/blob/1bb93f3ad258fdcc464c5e5c88f36005ae579ce0/README.md
I've tried to follow the general pattern in jupyterhub_config.py when naming LDAP parameters but not everything is clear to me.

LDAPAuthenticator seems to have many more parameters than the other auth providers, so I added a new function set_config_if_not_none in z2jh.py to make it easier to keep the built-in defaults from LDAPAuthenticator when a value isn't overridden.

I've only tested this with my own setup:

auth:
  type: ldap
  ldap:
    server:
      address: ldap.EXAMPLE.org
      port: 389
      ssl: False
    dn:
      templates:
        - 'cn={username},ou=edir,ou=people,ou=EXAMPLE-UNIT,o=EXAMPLE'
    allowedGroups:
      - 'cn=example-group,ou=groups,ou=EXAMPLE-UNIT,o=EXAMPLE'

Warning: I have not tested the other parameters, in particular the active directory setup from https://github.com/jupyterhub/ldapauthenticator/blob/1bb93f3ad258fdcc464c5e5c88f36005ae579ce0/README.md#active-directory-integration is untested by me.

[manics]

The equivalent of the example given in https://github.com/jupyterhub/ldapauthenticator/blob/1bb93f3ad258fdcc464c5e5c88f36005ae579ce0/README.md#active-directory-integration should be

auth:
  ldap:
    dn:
      lookup: true
      search:
        filter: '({login_attr}={login})'
        user: 'ldap_search_user_technical_account'
        password: 'secret'
        dnAttribute: 'cn'
      user:
        searchBase: 'ou=people,dc=wikimedia,dc=org'
        escape: False
        attribute: 'sAMAccountName'

[yuvipanda]

Thanks, @manics! I'll merge this once tests pass.

Can you add a doc page about this too? That'd be awesome!