As a follow up of
MDL-66612, some fallback icons are not matching the icons we are using on the fontawesome library.
In order to do that we should check the [icon library used as fontawesome|https://github.com/moodle/moodle/blob/master/lib/classes/output/icon_system_fontawesome.php,] create all fallback icons and replace them on the pix folder.
We should check icons height: Image icons are based mostly on width and font icons on height. We should get the same size on both versions. A great example to see that differences is the graduation cap used as course icon. When the fallback is used, the icon seem to be smaller. This is because this icon is wider and shorter.
One more thing to take into account is the icons used as background image: the fallback in SVG should have a background-size property to be correctly shown: ie graduation cap on the "Available courses" list on the main page will be displayed 2300+px wide (that will show only a white square).