refactor: Extract get_linkable_streams.

This is a one-liner with two purposes:

    * We want the comment to explain the business rule.

    * We want to just work in id space.

zerver/lib/mention.py

@@ -5,7 +5,7 @@ from typing import Dict, List, Match, Optional, Set, Tuple
 
 from django.db.models import Q
 
-from zerver.models import Realm, UserGroup, UserProfile, get_active_streams
+from zerver.models import Realm, UserGroup, UserProfile, get_linkable_streams
 
 # Match multi-word string between @** ** or match any one-word
 # sequences after @
@@ -206,8 +206,8 @@ def get_stream_name_map(realm: Realm, stream_names: Set[str]) -> Dict[str, int]:
     q_list = {Q(name=name) for name in stream_names}
 
     rows = (
-        get_active_streams(
-            realm=realm,
+        get_linkable_streams(
+            realm_id=realm.id,
         )
         .filter(
             functools.reduce(lambda a, b: a | b, q_list),

zerver/models.py

@@ -2476,6 +2476,16 @@ def get_active_streams(realm: Optional[Realm]) -> QuerySet:
     return Stream.objects.filter(realm=realm, deactivated=False)
 
 
+def get_linkable_streams(realm_id: int) -> QuerySet:
+    """
+    This returns the streams that we are allowed to linkify using
+    something like "#frontend" in our markup. For now the business
+    rule is that you can link any stream in the realm that hasn't
+    been deactivated (similar to how get_active_streams works).
+    """
+    return Stream.objects.filter(realm_id=realm_id, deactivated=False)
+
+
 def get_stream(stream_name: str, realm: Realm) -> Stream:
     """
     Callers that don't have a Realm object already available should use