Collapsible headings in app mode

[vangberg]

In edit mode it is possible to collapse all cells below a given heading level (implemented in #1826). It would be very helpful to have this feature in app mode as well, even better if it could be pre-configured, i.e. cells below a given header should default to being collapsed.

My specific use-case is that I add the user-facing UI at the top of the notebook, and the implementation at the bottom, under an "## Implementation" header. I like to be quite verbose, and add a bunch of Markdown cells explaining the implementation. It would be nice if that could be hidden by default when being run in app mode, as it clutters the UI.