Fixed #21231 -- Added mechanism to limit request data upload size.

Added settings.DATA_UPLOAD_MAX_MEMORY_SIZE, similar to the existing
FILE_UPLOAD_MAX_MEMORY_SIZE, and raised SuspiciousOperation on
requests that exceed it.
An unhandled `SuspiciousOperation` will result in a `400 Bad Request` response which is ok since it is a client error. However, `413 Request Entity Too Large` would be the correct status code for this error.