Slightly modernize NuanceSource

Summary:
Ref T8434. Minor cleanup/modernization. I made type selection modal (like Herald, Auth, etc) so we can render the form on the next screen based on the type.

Test Plan: Created a new source, edited an existing source.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8434

Differential Revision: https://secure.phabricator.com/D13161