Refactor folder structure

[thomcsmits]

Currently, there is a largely nested structure, with the actual template folder residing at src/user_templates_api/templates/jupyter_lab/templates/
This makes it tedious to find templates. It is also somewhat unintuitive where the inserted cells in src/user_templates_api/templates/jupyter_lab/utils/ and [...]/notebook are with regards to the src/user_templates_api/templates/utils/client functionality.

Preferred solution: fewer nested folders, with templates residing at e.g. src/templates/jupyter_lab, and all jupyter_lab functionality combined