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/Migrations/6.sql

@@ -1,3 +0,0 @@
--- SCHEMA VERSION 6: 2020-03-21
-alter table systems add column pings_enabled bool not null default true;
-update info set schema_version = 6;