[Apps] Data apps initial migration and endpoints (#24881)

* [Apps] Initial migrations and endpoints

Addresses #24879 and #24880, part of #24861

`App` is a container that brings various dashboards together into a
cohesive unit.

The field `nav_items` is used for sidebar navigation within the app.
The items in it will be managed by the front end and could contain
link definitions to dashboards or arbitrary urls.