Add super basic group model/command

PluralKit.Core/Database/Migrations/9.sql

@@ -0,0 +1,17 @@
+-- SCHEMA VERSION 9: 2020-xx-xx --
+
+create table groups (
+    id int primary key generated always as identity,
+    hid char(5) unique not null,
+    system int not null references systems(id) on delete cascade,
+    name text not null,
+    description text,
+    created timestamp with time zone not null default (current_timestamp at time zone 'utc')
+);
+
+create table group_members (
+    group_id int not null references groups(id) on delete cascade,
+    member_id int not null references members(id) on delete cascade
+);
+
+update info set schema_version = 9;