Use the page tree in the BE module

[oliverklee]

Story

As a developer, I want to have an example for a BE module that makes use of the page tree (including tests) so that I can learn how to write tests for a BE module with a page tree.

Acceptance criteria

• The BE module has the page tree enabled.
• The BE module only shows tea records from the selected page, not from other pages.
• If no page is selected in the page tree, a flash message is shown instead: "Please select a page in the page tree".