On group grid page: add button to create new group.

[HannahSonntag]

Currently there is no option to “Create a new group” on the group grid page.
We should add a button similar to the “Upload new SmartFigure”(the '+' on the dashboard) to the group grid page but with a group icon & a plus icon.

The icon I tried to immitate:

Upon clicking the button one is redirected to the usual 'create a new user group' page.