Allow navigation breadcrumbs to be marked as "always visible" so they show up on phones

Summary:
See PHI624. Some of the mobile navigation and breadcrumbs in support pacts aren't as good as they could be.

In particular, we generally collapse crumbs on mobile to just the first and last crumbs. The first crumb is the application; the last is the current page.

On /PHIxxx pages, the first crumb isn't very useful since the Support landing page is two levels up: you usually want to go back to the pact, not all the way back to the Support landing page.

We also don't need the space since the last crumb (PHIxxx) is always small.

Allow Support and other similar applications to tailor the crumb behavior more narrowly if they end up in situations like this.

Test Plan:

• With an additional change to instances (see next diff), viewed a support issue page (/PHI123) on mobile and desktop.
• Saw a link directly back to the pact on both mobile and desktop.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19438