Give "Auth Messages" a view/detail state before users customize them

Summary:
Depends on D20663. Ref T13343. Currently, if an Auth message hasn't been customized yet, clicking the message type takes you straight to an edit screen to create a message.

If an auth message has already been customized, you go to a detail screen instead.

Since there's no detail screen on the "create for the first time" flow, we don't have anywhere to put a more detailed description or a preview of a default value.

Add a view screen that works if a message is "empty" so we can add this stuff.

(The only reason we don't already have this is that it took a little work to build; this also generally improves the consistency and predictability of this interface.)

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13343

Differential Revision: https://secure.phabricator.com/D20664