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.

zerver/lib/topic_sqlalchemy.py

@@ -0,0 +1,20 @@
+from sqlalchemy.sql import ColumnElement, column, func, literal
+from sqlalchemy.types import Boolean, Text
+
+from zerver.lib.topic import RESOLVED_TOPIC_PREFIX
+
+
+def topic_match_sa(topic_name: str) -> ColumnElement[Boolean]:
+    # _sa is short for SQLAlchemy, which we use mostly for
+    # queries that search messages
+    topic_cond = func.upper(column("subject", Text)) == func.upper(literal(topic_name))
+    return topic_cond
+
+
+def get_resolved_topic_condition_sa() -> ColumnElement[Boolean]:
+    resolved_topic_cond = column("subject", Text).startswith(RESOLVED_TOPIC_PREFIX)
+    return resolved_topic_cond
+
+
+def topic_column_sa() -> ColumnElement[Text]:
+    return column("subject", Text)