Fix two minor breadcrumb issues in Config

Summary:
Fixes T13159. Two issues here:

• When viewing a particular config setting, there's an extra "Config" crumb.
• On the page for a config group, the link to the parent group has an extra "/config/" in it.

Test Plan:

• Viewed a page for a particular setting, no longer saw an extra "Config" crumb.
• Viewed a page for a setting group, clicked parent crumb, got taken to a real page.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13159

Differential Revision: https://secure.phabricator.com/D19501