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
Here's a failing test for #2872
gratipay.outbound -> gratipay.set_misc_headers

Change the name so it's more meaningful in tracebacks, and also remove
the dependency on `request`, which was trivial since the function
doesn't actually use it.
Fix test to actually test something testworthy
update authentication.{inbound,outbound}

authentication.inbound -> .read_auth_from_headers
authentication.outbound -> .add_auth_to_headers

And the latter no longer requires a request object.
Standardize nomenclature a little bit
Fix up csrf.{inbound,outbound}
Fix up cache_static.{inbound,outbound}