Per-guild presence sync settings

On by default for existing and small guilds.
Off for new large guilds.
Can be toggled either way.

src/db/migrations/0020-add-presence-to-guild-space.sql

@@ -0,0 +1,5 @@
+BEGIN TRANSACTION;
+
+ALTER TABLE guild_space ADD COLUMN presence INTEGER NOT NULL DEFAULT 1;
+
+COMMIT;