_frontend/app.py: Set correct element-path in interactive bst-init

Description

This MR fixes init_project in _frontend/app.py such that the element-path specified during an interactive bst init is correctly set

Changes proposed in this merge request:

• Set elements_path closer to the code where this directory path is actually created
• Add a regression test to check that element-path is correctly set during interactive bst init

This merge request, when approved, will close issue/bug: #717 (closed)