backups (tgz): work with a layout

this will allow us to place the localstorage directory in an arbitrary
location

migrations/20190114230513-apps-add-dataDir.js

@@ -0,0 +1,15 @@
+'use strict';
+
+exports.up = function(db, callback) {
+    db.runSql('ALTER TABLE apps ADD COLUMN dataDir VARCHAR(256) UNIQUE', function (error) {
+        if (error) console.error(error);
+        callback(error);
+    });
+};
+
+exports.down = function(db, callback) {
+    db.runSql('ALTER TABLE apps DROP COLUMN dataDir', function (error) {
+        if (error) console.error(error);
+        callback(error);
+    });
+};