Replace Aspen 0.33 with 0.35

Cleanup needed.
Update algorithm to keep up with Aspen changes
request.context is now a pure dict
Stop using request.website in error.spt
Remove request.website from n_get_text
Remove request.website from credit_card_expiring
Remove request.website from cache_static
Remove request.website from accounts elsewhere
Remove request.website from get_participant
Use aspen's new automatic json body parsing
Merge pull request #2877 from gratipay/aspen-0.35

upgrade from Aspen 0.33 to 0.35