tbdev turndown: add export converter script

[nfelt]

This adds a standalone script convert_tbdev_export.py that can convert the JSON-based export format for TensorBoard.dev (produced by tensorboard dev export) into tfevents files that can be used to display the exported data in open source TensorBoard. See documentation in the script itself for more details about how it's intended to be used.

Note that I've structured this as a standalone script in the style of diagnose_tensorboard.py (i.e. no BUILD target, no dependencies, no unit tests). Arguably, had we done this earlier, it would be better to have integrated this into the tensorboard dev export command itself as an output format option. But we're planning to remove the tensorboard dev command entirely once the turndown happens at EOY, so keeping it separate ensures that we can still link to this script for a while as a resource for anyone who has already exported their experiments to do conversions post-turndown.

Tested: did manual testing with a range of my own experiments, including some with multiple runs, different data types (scalars, tensors, blobs), etc. Confirmed that the conversion completed without errors and that it looked as expected in the TensorBoard UI. I've omitted unit tests because the manual end-to-end tests are what we ultimately care more about, and I don't expect any further significant development to happen for this script.