api_code_example: Use json instead of ujson; specify separators.
api_code_example: Use json instead of ujson; specify separators.

The Markdown extension that lives inside
zerver/lib/bugdown/api_code_example.py previously used ujson.
ujson's `dumps` function doesn't accept a `separators` argument,
which means we have no control over how the JSON is pretty-printed.
This resulted in JSON fixtures with no spaces after the colon, which
looks unnecessarily convoluted.

So now, we use the built-in `json` module to get around this.

For further reading, this issue
<https://github.com/esnme/ultrajson/issues/82> opened on ujson's
repo explains why they are reluctant to support such formatting
due to performance considerations.

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