Allow users to specify custom hostnames

database/migrations/05_add_custom_hostnames_flag_to_users_table.sql

@@ -0,0 +1 @@
+ALTER TABLE users ADD can_specify_hostnames BOOLEAN DEFAULT 1;

database/migrations/06_create_hostnames_table.sql

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