Fix broken UI tests

* UI tests were making plots outside the instance bounds (`make_instance` had a default radius of 300 units WebMercator)
* `make_instance` now takes a `radius` argument, and the UI tests set it to 10000
* Also simplify management tests by using the `radius` argument
* Clean up imports in UI tests
* Add `wait_until_visible` for safety in `_click_delete`
* Fix misplaced `json_api_edit` in `delete_tree_view`
Changed to `edge_length`, and the code now divides it by 2.