Add "Go to top" link when scroll down in boost and classic themes

Some teachers complain that when courses grow it's tedious to got to top page. This situation is even worse when a teacher is reviewing the course content and want to go to edit mode in the middle of the page.

To solve this situation I develop a small piece of javascript code and some CSS that detects when the user scrolls down and fixes a "Go to top" link and settings dropdown on the right site of the page (see attached image).

How does it works

The patch modifies these files:

• theme/boost/amd/src/aria.js (and minified): adds a class called "shrink" to the body when the user scrolls down ("shrink" is the standard name for this kind of dynamics because mainly is it used to shrink the top navbar)
•  theme/boost/templates/navbar.mustache (and classic theme equivalent): adds a go to top link
•  theme/classic/templates/navbar.mustache (and classic theme equivalent): adds a go to top link
• theme/boost/scss (and style): adding positions fixed when  user scrolls down
• lang/en/access.php: add a new "Go to top" string