Lightly modernize OAuth server application view pages

Summary: Depends on D20624. Fixes T13330. The OAuth client pages are using some out-of-date rendering conventions; update them to modern conventions.

Test Plan:
Viewed a page, saw a modern header layout + curtain:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13330

Differential Revision: https://secure.phabricator.com/D20625