Add webhook endpoints for guides

Whenever a guide/understanding doc/tutorial changes (i.e. new changes
have been pushed to the spring-guides repository), a webhook request is
sent by Github to the spring.io, which triggers a cache eviction for the
updated resource.

Note that Github signs webhook requests with an HMAC SHA-1 signature for
authentication, using a shared secret.

Modified as well the default expiration value for guides to never
expire, since Sagan is notifed whenever a change occured in the
repositories.

Fixes #467