Switch to using Lookup API instead of Ibis client. (#35)

Add settings for Lookup API and API Gateway credentials.

Add Lookup API operations to replicate the LDAP operations for gathering
information about users, groups and institutions.

Remove LDAP sync code and related configuration code.

Remove LDAP configuration options from the example file.

Remove LDAP library as a requirement it is no longer needed.

Remove LDAP references from documentation.

Add information to the README about how to provide API Gateway credentials.

Add YAML versions of API Gateway credentials file to `.gitignore`.