128000 and Lambda limits

[brianraymor]

https://github.com/HumanCellAtlas/matrix-service/blob/master/matrix/lambdas/api/core.py#L47

    if not has_url and len(json.dumps(body['bundle_fqids'])) > 128000:

128000 is a magic value that future maintainers may not understand.

Consider:

• Replacing the hardcoded value with a constant or variable that is self-documenting.
• Adding a comment that documents the relationship to limits on Lambda invocation payloads
• Updating the Swagger documentation to include the maximum size of bundle_fqids

[brianraymor]

Better documentation about the List of bundle fqids is too large. is implicit in #301.