Add a "Token" Credential type
Summary: Ref T9456. This is just a convenience type for things like API tokens, to make it harder for users to make mistakes and keep SSH keys out of the dropdown for "choose your API token".
Test Plan:
Reviewers: chad
Reviewed By: chad
Subscribers: joshuaspence
Maniphest Tasks: T9456
Differential Revision: https://secure.phabricator.com/D14284