zerver: Remove simplejson dependency.

Modified by tabbott to put the third-party code in a new file.

Fixes #6970.

zerver/lib/json_encoder_for_html.py

@@ -0,0 +1,28 @@
+import json
+
+# Taken from
+# https://github.com/simplejson/simplejson/blob/8edc82afcf6f7512b05fba32baa536fe756bd273/simplejson/encoder.py#L378-L402
+# License: MIT
+class JSONEncoderForHTML(json.JSONEncoder):
+    """An encoder that produces JSON safe to embed in HTML.
+    To embed JSON content in, say, a script tag on a web page, the
+    characters &, < and > should be escaped. They cannot be escaped
+    with the usual entities (e.g. &amp;) because they are not expanded
+    within <script> tags.
+    """
+
+    def encode(self, o):
+        # type: (Dict[str, Any]) -> str
+        # Override JSONEncoder.encode because it has hacks for
+        # performance that make things more complicated.
+        chunks = self.iterencode(o, True)
+        return ''.join(chunks)
+
+    def iterencode(self, o, _one_shot=False):
+        # type: (Dict[str, Any], Optional[bool]) -> Iterator[str]
+        chunks = super().iterencode(o, _one_shot)
+        for chunk in chunks:
+            chunk = chunk.replace('&', '\\u0026')
+            chunk = chunk.replace('<', '\\u003c')
+            chunk = chunk.replace('>', '\\u003e')
+            yield chunk