ldap: Add support for syncing custom profile fields from LDAP

[timabbott]

This shouldn't be too hard to develop and test; basically, we'd want to:

• For, review https://zulip.readthedocs.io/en/latest/subsystems/auth.html#testing-ldap-in-development for how to do LDAP testing in development.
• Update our generate_dev_ldap_dir function's mock LDAP directories to include values for some custom profile data fields (e.g. birthday would be a good choice).
• Add support for AUTH_LDAP_USER_ATTR_MAP containing CustomProfileFields, maybe spelled as custom_profile_field__birthday: "birthday",. To make this work, we'd need to add a _populate_user method in ZulipLDAPBackendBase in zproject/backends.py (overriding the default from django-auth-ldap (/srv/zulip-py3-venv/lib/python3.5/site-packages/django_auth_ldap/backend.py), though the it should call super() to get the built-in code to run). That should ensure that both manage.py sync_ldap_data and user creation sync the fields over properly.
• Document this is docs/production/authentication-methods.md.

Once this is done, two follow-ups are relevant:

• We should do jpegPhoto LDAP Integration #286 as well, since that'll involve the same technique (but with a bit more work since we need to deal with file uploads, so we should do this first).
• We should add options to control whether users can manually edit these custom profile fields (or perhaps just make them non-editable, since I think with this use case, that's almost certainly what you want)

[zulipbot]

Hello @zulip/server-authentication, @zulip/server-settings members, this issue was labeled with the "area: authentication", "area: settings (user)" labels, so you may want to check it out!

[HarshitOnGitHub]

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[zulipbot]

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[HarshitOnGitHub]

@zulipbot Was busy with some college stuff, will complete this by tomorrow. :)