Adjustements to external link, the title should tell the link opens a new window

Added functionality to give warnings when links open in new windows. ...

The solution is based on the recommendation in "Techniques for WCAG 2.0" section
"G201: Giving users advanced warning when opening a new window":
"Example 2: Using CSS to provide a warning before opening a new window"
(cf., https://www.w3.org/TR/WCAG20-TECHS/G201.html)

Changes:

- any link in the views with target="_blank" has had css class "has-new-window-popup-info" to the link together with the span   <span class="new-window-pop-info"><%= _('Opens in new window') %></span>.
- The javascript script externalLink.js is used for external links that don't have the target="_blank" in the view.

Fix for issue #2175.

moved JS tests to the spec dir and fixed issues with karma/webpack/jasmine ...

added lint and updated bundle task to use yarn

added lint and updated bundle task to use yarn

Update asset structure with webpacker gem