mute user: Cache list of muter IDs.

This commit defines a new function `get_muting_users`
which will return a list of IDs of users who have muted
a given user.
Whenever someone mutes/unmutes  a user, the cache will be
flushed, and subsequently when that user sends a message,
the cache will be populated with the list of people who
have muted them (maybe empty).

This data is a good candidate for caching because-

1. The function will later be called from the message send
codepath, and we try to minimize database queries there.

2. The entries will be pretty tiny.

3. The entries won't churn too much. An average user will
send messages much more frequently than get muted/unmuted,
and the first time penalty of hitting the db and populating
the cache should ideally get amortized by avoiding several
DB lookups on subsequent message sends.

The actual code to call this function will be written in
further commits.

zerver/lib/user_mutes.py

@@ -1,6 +1,7 @@
 import datetime
-from typing import Dict, List, Optional
+from typing import Dict, List, Optional, Set
 
+from zerver.lib.cache import cache_with_key, get_muting_users_cache_key
 from zerver.lib.timestamp import datetime_to_timestamp
 from zerver.models import MutedUser, UserProfile
 
@@ -34,3 +35,18 @@ def get_mute_object(user_profile: UserProfile, muted_user: UserProfile) -> Optio
         return MutedUser.objects.get(user_profile=user_profile, muted_user=muted_user)
     except MutedUser.DoesNotExist:
         return None
+
+
+@cache_with_key(get_muting_users_cache_key, timeout=3600 * 24 * 7)
+def get_muting_users(muted_user: UserProfile) -> Set[int]:
+    """
+    This is kind of the inverse of `get_user_mutes` above.
+    While `get_user_mutes` is mainly used for event system work,
+    this is used in the message send codepath, to get a list
+    of IDs of users who have muted a particular user.
+    The result will also include deactivated users.
+    """
+    rows = MutedUser.objects.filter(
+        muted_user=muted_user,
+    ).values("user_profile__id")
+    return {row["user_profile__id"] for row in rows}