feat: add new user settings backend

Adding a new user settings backend and PersistentStorage to store user related settings in the
database.

Signed-off-by: Dominik Schwank <dominik.schwank@sda.se>

plugins/user-settings-backend/.eslintrc.js

@@ -0,0 +1 @@
+module.exports = require('@backstage/cli/config/eslint-factory')(__dirname);