Add icon for entity synchronization

Problem/Motivation

The Entity Synchronization → framework provides a top-level menu item and an icon for the admin toolbar. It would be nice to have that with Gin as well. The icon is a "refresh" icon from font-awesome, I'm posting a screenshot.

Remaining tasks

I gave it a go, but I'm not a frontend developer and I didn't have good luck with creating the SVG and adding it to the sprite. I'm adding an MR with the rest, if somebody could help with it that would be great.