Support authenticated connections

[rjw57]

Currently the lookup connection is created in ucamlookup.utils by simply calling createConnection. Extend this to allow configuration of the connection by Django settings. This will allow using a group's credentials to authenticate.

Suggested settings:

• UCAMLOOKUP_HOST. Optional. Default: "www.lookup.cam.ac.uk"
• UCAMLOOKUP_PORT. Optional. Default: 443
• UCAMLOOKUP_URL_BASE. Optional. Default: ""
• UCAMLOOKUP_CHECK_CERTS. Optional. Default: True
• UCAMLOOKUP_USERNAME. Optional. Default: None
• UCAMLOOKUP_PASSWORD. Optional. Default: None.

Apart from the username and password settings, these are passed directly to the IbisClientConnection constructor. If non-None, the username is set on the connection via the set_username method. If non-None, the password is set on the connection via the set_password method.

[rjw57]

I've just had verbal confirmation from AIS that global authenticated access to Lookup should go live by Match 7th 2019.