Presentation of rich-text fields in editor for translated pages

[nimasmi]

This is also related to #247

This is feedback from a client who is using wagtail-localize:

Translators tend to lose the notion of the text's structure, because every block-level element is displayed the same way. In long structured texts with different heading levels and bullet points, this can become quite confusing for translators. Perhaps it could be improved just by using specific default styling for each HTML element.

e.g.

A list

• list element 1
• list element 2

Is presented as three equal-status inputs, and the context is lost.