Allow users to reserve subdomains (#131)

database/migrations/04_create_subdomains_table.sql

@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS subdomains (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id INTEGER NOT NULL,
+    subdomain STRING NOT NULL,
+    created_at DATETIME,
+    updated_at DATETIME
+)