LDAP account mapper

## Problem/Opportunity Statement

We'd like to be able to map Github users to LC users. Usernames will be stored in the `githubId` field.

## What would success / a fix look like?

Write an account mapper that queries an arbitrary LDAP endpoint (and potentially a custom field to map to).

TODO:
- https://github.com/python-ldap/python-ldap
- look at lc-idm source