In "External Accounts", replace hard-to-find tiny "link" icon with a nice button with text on it

Summary:
Ref T6703. Replaces the small "link" icon with a more obvious "Link External Account" button.

Moves us toward operating against $config objects instead of against $provider objects, which is more modern and will some day allow us to resolve T6703.

Test Plan: Viewed page, saw a more obvious button. Linked an external account.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T6703

Differential Revision: https://secure.phabricator.com/D20105