Replace the usage of a single application-wide session crypter
with per-session crypters.
The application is no longer able to decrypt any session
encrypted with its symmetric key alone. Instead, a session ticket
with its associated data encryption key (DEK) is also required in order
to decrypt the associated session data. The ticket itself is
encrypted with the application's crypter; the latter of which is
effectively a key-encryption key (KEK).
Fixes#49.
Replace hardcoded callback URLs with dynamic generation
of URLs based on incoming requests. These are validated against
a pre-registered list of ingresses for which Wonderwall is considered
authorative for.
We also preserve the cookie behaviour; the most specific ingress path
and domain is used for the cookies.
The `url` package has been moved to the `handler` package, and its
implementation refactored slightly for readability and DRY.
