[ENH] Update AppScript, move to new JB build system

[emdupre]

⚠️ This is currently a draft PR ! ⚠️

This PR provides several small updates in style, documentation, and functionality; though its major contribution will be to move to the new JupyterBook build system.

Other changes of note:

• Moves from a "blocked" list to an "allowed" list. Only pre-cleared authors will be allowed to submit to NeuroLibre.
• Do not lock new GitHub issue by default. The current workflow is too fast to warrant a locked issue period.
• Removes requirement for a Jupyter Notebook. Jupyter Book can handle .ipynb, md, or .rst content types.