Automatically expand all textareas #1862

Having to scroll a 200px-tall textarea with pasted-content (code, logs) is awful.

GitHub actually fits most comment boxes to their content, up to a limit. I looked into the code and there's no way to remove the limit, but thankfully we just have to drop the js-size-to-fit class and use fit-textarea:

<img width="361" src="https://user-images.githubusercontent.com/1402241/54336211-66fd5e00-4666-11e9-9c5e-111fccab004d.gif">

Same applies to CodeMirror instances, but the change is probably a matter applying some CSS (as part of the same JS feature)