Port UserRebaseConfirmable to Go

UserRebaseConfirmable is currently handled by the Gitaly-Ruby sidecar, and should be ported to Go for performance reasons. This could be handled by shelling out to Git, leveraging Git2Go, or another way.

Edited Sep 22, 2020 by Toon Claes

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information