Collapse/expand long files

[cayhorstmann]

When a very long file is displayed, e.g. https://horstmann.com/sjsu/cs046/lesson4-35.html, show the first few lines and provide a button to expand or collapse it.

[cayhorstmann]

TIP: This is easier to observe on https://codecheck.io/files/cs046/lesson4-book2. The display is a sequence of "Ace editor" elements, generated by https://codecheck.io/assets/horstmann_codecheck.js. The suggestion is to put a button below the editor when the contents is too large. Do we need to swap the text upon button click? Or can the Ace editor help? I see something about minLines/maxLines in the code.

[cayhorstmann]

TIP: To create a button, use the helper method createButton in util.js. You find example usages in horstmann_codecheck.js.

createButton('hc-command', 'Expand', () => { ... })

Put the commands that should be executed into the braces.