Integration of tree navigation into the test service page

*** Please read this before merging ***

Please note that the tree navigation requires the list services in order for it to work:
/api/1/content/asset/list/{site} => lists all assets in the root level
/api/1/descriptor/list/{site} => lists all descriptors in the root level
/api/1/template/list/{site} => lists all templates in the root level

Likewise, it's possible to get all assets for a specific folder using the item_id parameter in the URL.
For example,
/api/1/content/asset/list/{site}?item_id={node_id} => lists all first-level assets under the folder with id={node_id}

Additionally, the config files returned by the backend should be:

/api/1/config/list/studio-ui:
https://github.com/quiaro/studio3/blob/test-asset-service/ui/server/app/mocks/config/list/studio-ui.json

/api/1/config/list/test-service:
https://github.com/quiaro/studio3/blob/test-asset-service/ui/server/app/mocks/config/list/test-service.json

/api/1/config/list/tree-navigation:
https://github.com/quiaro/studio3/blob/test-asset-service/ui/server/app/mocks/config/list/tree-navigation.json

This code will not work in the test server until the list services have been implemented so please accept/merge this PR at your own discretion.