Show more information about OAuth tokens in the Account Settings -> External Accounts screen

Summary:
Ref T1536.

  - Allow providers to customize the look of external accounts.
  - For username/password auth, don't show the account view (it's confusing and not useful).
  - For OAuth accounts, show token status.

Test Plan:
{F47374}

{F47375}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6289