sentry: send envelopes, add drkonqi-sentry-postman, add tests

this sees some major refactoring to how submissions work.

previously we sent the crash event followed by the user feedback event
to their respective ingestion points. sentry does however also feature
an alternative format where we can send multiple payloads in the same
call. this specifically also allows us to store an entire "submission"
in a single file without having to invent anything ourselves.

consequently sentrybeacon is no more, it's been replaced by
sentrypostbox which now constructs/manages a sentryenvelope which is
representing that new storage format.

an envelope is comprised of a header and payload. the payload are
multiple "items" of which each again has a header and a payload of their
own. whenever the envelope changes it also updates itself in the
sentryenvelopecache.

the cache acts as file-based storage system. the envelope is pushed into
the cache which results in a write to
~/.cache/drkonqi/sentry-envelopes/. eventually, ...