Offer Ubuntu customizations in new user mode too

GNOME Initial Setup offers two modes, existing user and new user mode. Pop!_OS and Fedora (new in next months' Fedora 28 release) use the new user mode. The new user mode runs full screen in GDM when there are no non-system users on the computer. It adds two additional pages: a user name page and a password page.

In my testing at the end of the wizard, it logged me in (without needing to pick my name and enter my password yet again) and loaded the GNOME Getting Started help.

I am attaching a small patch to apply against gnome-initial-setup 3.28.0-2ubuntu1 that uses the new Ubuntu version of gnome-initial-setup instead of the GNOME version in the new user mode.

I am not proposing that Ubuntu switch to new user mode by default in 18.04. It's an interesting option that could be useful to some OEMs as an alternative to ubiquity's OEM mode.

I am attaching screenshots so you can get an idea of how this works without needing to go through the effort of trying it out.

Try It Out
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(on a virtual machine or where you have backed up all your home directories!)
- Start Ubuntu 18.04 into recovery mode. (Hold Shift when starting and select the option in grub).
- Select clean. (This will run apt autoremove, but more importantly, it will mount / read-write).
- Select root. Press enter to reach the command prompt
- Run. (Replace 'jeremy' with your username. Repeat for every user that would show up in gdm's login window).
sudo deluser --remove-home jeremy
exit
- Select resume.

This should now start the New User mode.

Issues
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1. The theme is using Adwaita instead of Ambiance
The home directory for the gnome-initial-setup system user is set to /run/gnome-initial-setup.

2. My patch feels a bit of a hack since it also forces other flavors to use the Ubuntu version for gnome-initial-setup and wouldn't be acceptable as is upstream. But "vanilla GNOME" doesn't make an ISO and Pop!_OS uses a custom pop-gnome-initial-setup anyway.

Perhaps we could have gnome-initial-setup in new user mode set XDG_CURRENT_DESKTOP=Ubuntu. That would fix the theme problem. Or we could maybe have it call the gsettings command to set the themes and fonts before it runs the main binary.

3. The Privacy link doesn't work. We should switch it to using a webkit-powered popup like upstream does for their privacy link.

4. We need to hide the "Open Software now" link in new user mode. It actually does work and opens the Software app which is not a good idea here.

5. The Livepatch dialog doesn't open.

6. I guess we should add "stylesheetName": "ubuntu.css", to /usr/share/gnome-shell/modes/initial-setup.json