Serialize data as utf-8-escaped US-ascii-JSON.

Data from 3 sources is encoded as utf-8 code-point, US-ascii-escaped,
JSON-serialized objects, prior to transmission from the webserver to
the flappserver.

The data is:
nickname, and PGP public key from the HTTP POST payload
customer(dot) attributes from stripe
log and secret file names from the local filepath objects.