Revert Button for OS side-by-side installation needed

[brs17]

If this is a bug, please use the template below. If this is a feature request, question, or a general discussion topic - please post on our subreddit https://reddit.com/r/pop_os - as that is the proper forum for those types of posts.

Reporter info

 * Distribution - (run ```cat /etc/os-release```)
 * Related Application and/or Package Version - apt policy $PACKAGE NAME

NAME="Pop!_OS" VERSION="17.10 (Artful Aardvark)" BUILD = 31 (noNVIDIA)

Issue/Bug Description In the Ubiquity installer, when selecting to install Pop!_OS alongside another OS, a graphical tool is shown asking the user to customize how much space each OS should take up on the drive. In the advance partitioning tool, there is a revert button in case a change is made that the user did not want done. This does not exist in the simple setup. While the "back" button does undo all changes, a Revert button should be added to simplify and match with the more advance partitioning tool. The page I am detailing is shown in the image below.

Steps to reproduce (if you know) Run the installer with an OS already installed on the harddrive.

Expected behaviour

Other Notes

[isantop]

This issue was moved to pop-os/pop#27