schema.sql: change roles.color default to 0

fixes the at-everyone role being black.

related to !6.

 - migration.scripts: add 8_roles_default_color

manage/cmd/migration/scripts/8_roles_default_color.sql

@@ -0,0 +1,9 @@
+-- update roles.color default to 0
+ALTER TABLE roles
+  ALTER COLUMN color SET DEFAULT 0;
+
+-- update all existing guild default roles to
+-- color=0
+UPDATE roles
+  SET color = 0
+WHERE roles.id = roles.guild_id;

schema.sql

@@ -519,7 +519,7 @@ CREATE TABLE IF NOT EXISTS roles (
     guild_id bigint REFERENCES guilds (id) ON DELETE CASCADE,
 
     name text NOT NULL,
-    color int DEFAULT 1,
+    color int DEFAULT 0,
     hoist bool DEFAULT false,
     position int NOT NULL,
     permissions int NOT NULL,