Show news in Studio welcome page

In order to better inform people about changes in Nuxeo Studio

When I visit the Studio homepage in Modeler:

• A polymer element is used to display the product news created using the backend described in NXCONNECT-2111
• The existing iframe is removed

When I visit the Studio homepage in Designer:

• The same polymer element is used (goal here is to be able to put different news on each side, so that info is there when it makes sense)
• Current message

  Select on the left panel the template you want to adapt. Then, drop new elements or configure properties.

  is replaced by this element