Add WebStatus authz "view" action.

This commit adds a new action to authz called "view". This action
represents the ability to view the Buildbot's WebStatus. If, for
example, you set `view='auth'` in authz, a user must be logged in to
view any meaningful status information.

The exception is that the root page and the login action are
whitelisted to always be viewable since these are necessary to log in.

Using this authz action it becomes possible to run a "private" Buildbot
instance for projects that don't wish to expose themselves to the
Internet at large.