Add collapsible code blocks

I do network recon and use Boostnote to track the progress and results. That often involves pasting command line output and device config files into the note as code blocks. These blocks, especially the configs, can be hundreds of lines long.

It would make the note much more readable if code blocks were collapsible when viewing the note, especially if blocks over a certain length (10-15 lines) defaulted to being collapsed.

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