Reset view paths after request

[jasonvarga]

Fixes statamic/ssg#116
Related statamic/ssg#119

This PR will reset the view paths and namespace hints after the request completes. This prevents them from leaking into other requests, for example in tests or in the SSG.

The tests are tweaked so that the assertion happens in the closure (simulating within the context of a request).

The middleware is also refactored so that the handle method is more readable.

[what-the-diff]

• The AddViewPaths middleware was refactored to be more testable.
• A new method, updateHints(), was added that updates the view hints for namespaced views based on the current site handle and locale code (if applicable).
• Tests were updated accordingly to reflect these changes in functionality and structure of this class/middleware.