Too many refactors in one:

- Allowed adding ephemeral(ish) views and functions
- Moved message_count to a concrete database field
- Moved most proxy logic to a stored procedure
- Moved database files around and refactored schema manager

PluralKit.Core/Database/clean.sql

@@ -0,0 +1,3 @@
+drop view if exists system_last_switch;
+drop view if exists member_list;
+drop function if exists proxy_info;