Self-text expando max height

Relates to #1497.

I'm not sure how feasible it is to add the same kind of option for comment max height (as requested), since we'll have to deal with inline expandos (i.e. when an expando is opened, override the max-height style in the current .usertext-body element, and when an expando is closed check if it is the last one in this comment to be closed [preferably: check if closing it will reduce the comment's height to within the specified max height] and remove the override).
I don't know if that much work would be worth it.
we should sanitize/validate this value:

    var selfTextMaxHeight = parseInt(this.options.selfTextMaxHeight.value, 10);
    if (selfTextMaxHeight) { RESUtils.addCSS(' ... ' + selfTextMaxHeight + 'px; ...'); }