This reverts course on 4201fbd778, and restores event.schema.json from that
commit. In that commit we said: 'this is not used'. Not true: it's used in a
test, though this test used the validity check to silently skip.
In this commit:
1. Do _not_ just silently skip invalid samples. Since we have a way of properly
validating, let's use that so that we know how useful the samples that we have
actually are.
2. Deal with "_meta", a field that we sometimes see in the "private samples" (data
that ultimately comes from running a somewhat recent python-sdk against my
actual codebase). The need for this was exposed by [1]
3. Add a test for the up-to-date-ness of event.json.schema
4. remove special-cased attribute-checks in `is_valid`; `send_json` was, at the
time, an opportunistic way to just get my hands on some sample data. the
approach at validation reflected that: I just did some tests on the existence
of certain attributes to determine which json files were even events. But in
the end I did a full validation using an API schema, which kinda made the
whole business useless. This commit cleans up the individual checks.
