Adding static pages action for 502

Instead of a true static page, the /502(.html) route is sent to the
StaticPages#timeout_error. This allows for the generation of a
form with the CSRF token.

Resolves ndlib/curate#29 without a need to merge this pull request.

@TODO - This means that the HelpRequest model needs adjustment within
the local instance of CurateND. The 502 error may not have a current
user. Which means we need a fallback email to be the sender of the
help request.