Read tree ID from detail page tree section header rather than field

[jwalgran]

Overview

Instance managers can hide the tree ID field from view, which was breaking the delete logic.

Connects #3265 Connects https://github.com/OpenTreeMap/otm-clients/issues/439 Depends on https://github.com/OpenTreeMap/otm-cloud/pull/467

Testing Instructions

• Create an instance named delete-without-tree-id-test
• Browse http://localhost:6060/delete-without-tree-id-test/management/field-configuration/
• Click "Edit"
• Uncheck "ID" in the "Tree" section.
• Click "Save"
• Click "Add a tree." Complete the add tree process, filling in a species and diameter.
• Browse the detail page for the tree
• Click "Delete" and "Confirm Deletion". Verify that the tree is deleted without error.

[coveralls]

Coverage remained the same at 83.796% when pulling 77fd0507234f725c69a8c9cf8e142edb54e36072 on jcw/delete-tree-with-hidden-id into dc0428c468707a38ea41176204b527b1832e9a8d on hotfix/2.21.5.

[jwalgran]

Thanks.