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/lib/user_mutes.py

@@ -0,0 +1,43 @@
+import datetime
+from typing import Dict, List, Optional
+
+from django.utils.timezone import now as timezone_now
+
+from zerver.lib.timestamp import datetime_to_timestamp
+from zerver.models import MutedUser, UserProfile
+
+
+def get_user_mutes(user_profile: UserProfile) -> List[Dict[str, int]]:
+    rows = MutedUser.objects.filter(user_profile=user_profile).values(
+        "muted_user__id",
+        "date_muted",
+    )
+    return [
+        {
+            "id": row["muted_user__id"],
+            "timestamp": datetime_to_timestamp(row["date_muted"]),
+        }
+        for row in rows
+    ]
+
+
+def add_user_mute(
+    user_profile: UserProfile,
+    muted_user: UserProfile,
+    date_muted: Optional[datetime.datetime] = None,
+) -> None:
+    if date_muted is None:
+        date_muted = timezone_now()
+    MutedUser.objects.create(
+        user_profile=user_profile,
+        muted_user=muted_user,
+        date_muted=date_muted,
+    )
+
+
+def remove_user_mute(user_profile: UserProfile, muted_user: UserProfile) -> None:
+    MutedUser.objects.get(user_profile=user_profile, muted_user=muted_user).delete()
+
+
+def user_is_muted(user_profile: UserProfile, muted_user: UserProfile) -> bool:
+    return MutedUser.objects.filter(user_profile=user_profile, muted_user=muted_user).exists()