export: Treat is_mirror_dummy=True users as consenting.

As explained in the comment added to the function, in terms of privacy
concerns, it is fine to export all data for these accounts. And it is
important to do - so that exporting an organization which was originally
imported e.g. from Slack doesn't result in excessively limited data for
accounts that were mirror dummies and never "activated" themselves.

zerver/lib/export.py

@@ -1214,7 +1214,7 @@ def custom_fetch_user_profile(response: TableData, context: Context) -> None:
     for row in rows:
         if exportable_user_ids is not None:
             if row["id"] in exportable_user_ids:
-                assert not row["is_mirror_dummy"]
+                pass
             else:
                 # Convert non-exportable users to
                 # inactive is_mirror_dummy users.
@@ -2713,6 +2713,16 @@ def get_consented_user_ids(realm: Realm) -> set[int]:
     # 1) It is a human account and enabled allow_private_data_export.
     # 2) It is a bot account with allow_private_data_export toggled on.
     # 3) It is a bot whose owner is (1).
+    # 4) It is a mirror dummy. This is a special case that requires some
+    #    explanation. There are two cases where an account will be a mirror dummy:
+    #    a) It comes from a 3rd party export (e.g. from Slack) - in some cases,
+    #       certain limited accounts are turned into Zulip mirror dummy accounts.
+    #       For such an account, the admins already have access to all the original data,
+    #       so we can freely consider the user as consenting and export everything.
+    #    b) It was imported from another Zulip export; and it was a non-consented user
+    #       in it. Thus, only public data of the user was exported->imported.
+    #       Therefore, again we can consider the user as consenting and export
+    #       everything - all this data is public by construction.
 
     query = sql.SQL("""
         WITH consenting_humans AS (
@@ -2732,6 +2742,7 @@ def get_consented_user_ids(realm: Realm) -> set[int]:
                 AND is_bot
                 AND realm_id = {realm_id}
             )
+            OR (is_mirror_dummy AND realm_id = {realm_id})
     """).format(realm_id=sql.Literal(realm.id))
 
     with connection.cursor() as cursor: