Extended description in form fields

[jace]

Form fields currently accept a description that is expected to be a single line. Sometimes we need an extended description spanning paragraphs. This can't be displayed inline and will be a pop-up.

This needs two things:

1. All Field classes offered by Baseframe should accept an extended_description or long_description parameter. Possible values: markup text, name of template to render, or URL that will supply the content. This is because the description may require non-textual content like icons and images.

2. render_field should render the field with an icon indicating extra content is available. This should show a tooltip or popup with the content.