db [nfc]: Use step-by-step migration helper

It is a thin wrapper that does what we are already doing in our
migration code.

While not required, an advantage of this is that it causes compilation
errors for missing migration steps.

Author: PIG208

lib/model/database.dart

@@ -86,13 +86,13 @@ class AppDatabase extends _$AppDatabase {
         }
         assert(1 <= from && from <= to && to <= schemaVersion);
 
-        if (from < 2 && 2 <= to) {
-          final schema = Schema2(database: m.database);
-          await m.addColumn(schema.accounts, schema.accounts.ackedPushToken);
-        }
-        // New migrations go here.
-      }
-    );
+        await m.runMigrationSteps(from: from, to: to,
+          steps: migrationSteps(
+            from1To2: (m, schema) async {
+              await m.addColumn(schema.accounts, schema.accounts.ackedPushToken);
+            },
+          ));
+      });
   }
 
   Future<int> createAccount(AccountsCompanion values) async {