Integrate Salesforce API search into the /users search form

[ragesoss]

Current Behavior:

The form at /users lets an admin search for users by real name or email address. However, many instructors use multiple email addresses, and may contact a Wiki Education staff member via an email address that is different from the one associated with their account on the Dashboard. In many of these cases, we have their other email address in Salesforce.

Desired Behavior:

As a fallback, when no user is found for an email or real name search on the Dashboard, we should query for a Contact record against the Salesforce API.

Additional context:

This will need to be implemented by Sage or by a contractor whom we can provide Salesforce API access for testing.