N°3807 Bring back styling to raw HTML content

Bulma minireset (https://github.com/jgthms/bulma/blob/master/sass/base/minireset.sass) was applied everywhere in iTop. This was causing HTML content without any Bulma or iTop 3.0.* CSS classes to render with no styles anymore, not even the default browser's ones. Especially rendering for content styled in CK Editor was problematic...

This commit creates a new `ibo-is-html-content` CSS class (in css/backoffice/utils/helpers/_misc.scss) that just extends the `content` Bulma class (indirection to reduce framework coupling).
This new iTop CSS class is added in :
* AttributeText and its children when format is HTML
* HTML components
* activity entries in HTML format

The class can also be used elsewhere when needed, for example in modules having custom pages that aren't using yet the iTop 3.0.* UI components or CSS classes.

css/backoffice/utils/helpers/_misc.scss

@@ -142,6 +142,14 @@ body.ibo-has-fullscreen-descendant {
   @extend %ibo-font-code-150;
 }
 
+/*
+ * A single class to handle WYSIWYG generated content, where only HTML tags are available
+ * See https://bulma.io/documentation/elements/content/
+ */
+.ibo-is-html-content {
+  @extend .content;
+}
+
 /***********************************************************************/
 /* Sticky headers                                                      */
 /*                                                                     */