Followup on https://github.com/go-gitea/gitea/pull/15952, use SVG for migration icon. <img width="541" alt="Screen Shot 2021-05-23 at 00 26 12" src="https://user-images.githubusercontent.com/115237/119242417-c1a37600-bb5d-11eb-9f97-a80aa89741ee.png"> <img width="540" alt="Screen Shot 2021-05-23 at 00 25 39" src="https://user-images.githubusercontent.com/115237/119242419-c2d4a300-bb5d-11eb-9792-1e6e4092c9f9.png">
silverwind
