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/Migrations/4.sql

@@ -0,0 +1,3 @@
+-- SCHEMA VERSION 4: 2020-02-12
+alter table member_guild add column avatar_url text;
+update info set schema_version = 4;