Avoid nested paragraphs in description

[PhilippeVay]

A multi-block description already creates as many paragraphs as there are "blocks" of description and the partial view section.html nests it in a parent paragraph, which is invalid HTML and may cause several problem (with browser parsers, with screen-readers and other assistive technologies).

Changing container .sg-section-description for a div avoids nested p.

Example with this 3 paragraphs-long description:

// Title of section
//
// Paragraph 1
//
// Paragraph 2
//
// Paragraph 3
//
// markup: (…)

would now result in:

<div ng-bind-html="section.description |&nbsp;unsafe" class="sg sg-section-description ng-binding">
  <p class="sg">Paragraph 1</p>
  <p class="sg">Paragraph 2</p>
  <p class="sg">Paragraph 3</p>
</div>

With this modification, single-line descriptions are still parsed as a single paragraph p.sg and have this div as a container so both cases (single and multi-paragraphs descriptions) are fine.