topic: Move sqlalchemy methods into their own file.

Loading sqlalchemy can take a significant amount of time, so splitting
these into these own file can be a significant startup-time savings.

tools/linter_lib/custom_check.py

@@ -13,6 +13,7 @@ from zulint.custom_rules import Rule, RuleList
 FILES_WITH_LEGACY_SUBJECT = {
     # This basically requires a big DB migration:
     "zerver/lib/topic.py",
+    "zerver/lib/topic_sqlalchemy.py",
     # This is for backward compatibility.
     "zerver/tests/test_legacy_subject.py",
     # Other migration-related changes require extreme care.