refactor: Break up build_subscriptions.

Having two smaller functions should make it
easier to customize the behavior for each specific
use case.  The only reason they were ever coupled
was to keep ids in sequence, but the recent NEXT_ID
changes make that a non-issue now.

zerver/data_import/hipchat.py

@@ -29,7 +29,8 @@ from zerver.data_import.import_util import (
     build_realm_emoji,
     build_recipients,
     build_stream,
-    build_subscriptions,
+    build_personal_subscriptions,
+    build_public_stream_subscriptions,
     build_user_message,
     build_user_profile,
     build_zerver_realm,
@@ -672,11 +673,17 @@ def do_convert_data(input_tar_file: str, output_dir: str) -> None:
     )
     realm['zerver_recipient'] = zerver_recipient
 
-    zerver_subscription = build_subscriptions(
+    public_stream_subscriptions = build_public_stream_subscriptions(
         zerver_userprofile=normal_users,
         zerver_recipient=zerver_recipient,
         zerver_stream=zerver_stream,
     )
+
+    personal_subscriptions = build_personal_subscriptions(
+        zerver_recipient=zerver_recipient,
+    )
+
+    zerver_subscription = public_stream_subscriptions + personal_subscriptions
     realm['zerver_subscription'] = zerver_subscription
 
     zerver_realmemoji = write_emoticon_data(

zerver/data_import/import_util.py

@@ -98,7 +98,7 @@ def build_subscription(recipient_id: int, user_id: int,
     subscription_dict['recipient'] = recipient_id
     return subscription_dict
 
-def build_subscriptions(
+def build_public_stream_subscriptions(
         zerver_userprofile: List[ZerverFieldsT],
         zerver_recipient: List[ZerverFieldsT],
         zerver_stream: List[ZerverFieldsT]) -> List[ZerverFieldsT]:
@@ -108,11 +108,6 @@ def build_subscriptions(
     the Hipchat export, so this function just autosubscribes all
     users to every public stream.  This returns a list of Subscription
     dicts.
-
-    This function also creates personal subscriptions.
-
-    If you need more fine tuning on how to subscribe folks, look
-    at the code in slack.py.
     '''
     subscriptions = []  # type: List[ZerverFieldsT]
 
@@ -143,6 +138,12 @@ def build_subscriptions(
             )
             subscriptions.append(subscription)
 
+    return subscriptions
+
+def build_personal_subscriptions(zerver_recipient: List[ZerverFieldsT]) -> List[ZerverFieldsT]:
+
+    subscriptions = []  # type: List[ZerverFieldsT]
+
     personal_recipients = [
         recipient
         for recipient in zerver_recipient