mute user: Add backend infrastructure code.

Adds backend code for the mute users feature.
This is just infrastructure work (database
interactions, helpers, tests, events, API docs
etc) and does not involve any behavioral/semantic
aspects of muted users.

Adds POST and DELETE endpoints, to keep the
URL scheme mostly consistent in terms of `users/me`.

TODOs:
1. Add tests for exporting `zulip_muteduser` database table.
2. Add dedicated methods to python-zulip-api to be used
   in place of the current `client.call_endpoint` implementation.

zerver/openapi/python_examples.py

@@ -601,6 +601,32 @@ def toggle_mute_topic(client: Client) -> None:
     validate_against_openapi_schema(result, "/users/me/subscriptions/muted_topics", "patch", "200")
 
 
+@openapi_test_function("/users/me/muted_users/{muted_user_id}:post")
+def add_user_mute(client: Client) -> None:
+    ensure_users([10], ["hamlet"])
+    # {code_example|start}
+    # Mute user with ID 10
+    muted_user_id = 10
+    result = client.call_endpoint(url=f"/users/me/muted_users/{muted_user_id}", method="POST")
+    # {code_example|end}
+
+    validate_against_openapi_schema(result, "/users/me/muted_users/{muted_user_id}", "post", "200")
+
+
+@openapi_test_function("/users/me/muted_users/{muted_user_id}:delete")
+def remove_user_mute(client: Client) -> None:
+    ensure_users([10], ["hamlet"])
+    # {code_example|start}
+    # Unmute user with ID 10
+    muted_user_id = 10
+    result = client.call_endpoint(url=f"/users/me/muted_users/{muted_user_id}", method="DELETE")
+    # {code_example|end}
+
+    validate_against_openapi_schema(
+        result, "/users/me/muted_users/{muted_user_id}", "delete", "200"
+    )
+
+
 @openapi_test_function("/mark_all_as_read:post")
 def mark_all_as_read(client: Client) -> None:
 
@@ -1352,6 +1378,8 @@ def test_users(client: Client, owner_client: Client) -> None:
     add_alert_words(client)
     remove_alert_words(client)
     deactivate_own_user(client, owner_client)
+    add_user_mute(client)
+    remove_user_mute(client)
 
 
 def test_streams(client: Client, nonadmin_client: Client) -> None: