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.

zerver/lib/bugdown/api_code_examples.py

@@ -1,7 +1,7 @@
 import re
 import os
 import sys
-import ujson
+import json
 import inspect
 
 from markdown.extensions import Extension
@@ -143,7 +143,8 @@ class APICodeExamplesPreprocessor(Preprocessor):
         else:
             fixture_dict = zerver.lib.api_test_helpers.FIXTURES[function]
 
-        fixture_json = ujson.dumps(fixture_dict, indent=4, sort_keys=True)
+        fixture_json = json.dumps(fixture_dict, indent=4, sort_keys=True,
+                                  separators=(',', ': '))
 
         fixture.append('```')
         fixture.extend(fixture_json.splitlines())