Add basic database schema migration system

PluralKit.Core/Migrations/1.sql

@@ -0,0 +1,15 @@
+-- SCHEMA VERSION 1: 2019-12-26
+-- First version introducing the migration system, therefore we add the info/version table
+
+create table info
+(
+    id int primary key not null default 1, -- enforced only equal to 1
+    
+    schema_version int,    
+    
+    constraint singleton check (id = 1) -- enforce singleton table/row
+);
+
+-- We do an insert here since we *just* added the table
+-- Future migrations should do an update at the end
+insert into info (schema_version) values (1);