Use gnome-initial-setup to replace 2nd half of ubiquity

This is a proposal for the Ubuntu GNOME installer (ubiquity) to skip directly from the partition page to the slideshow.

On first boot, gnome-initial-setup recognizes that no valid user has been set up and runs in New User mode to create that first user. It asks questions to set the language, keyboard layout, and timezone. It asks for the username, display name and password and optionally allows picking a login avatar (or 'face') and setting up Online Accounts.

It's too late to do this for 17.04 but maybe we could do it for 17.10.

To Do
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- We probably need to fix gnome-initial-setup's language page first (LP: #1614816)

- Then get ubiquity to skip the extra questions for Ubuntu GNOME

- How will we handle setting the hostname?

To try this
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(on a virtual machine or where you have backed up all your home directories!)
- Start Ubuntu GNOME 17.04 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.
- 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.

Why switch?
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- Better integration with GNOME
- Easy for OEMs or anyone who wants to install Ubuntu GNOME for someone else. They just need to do the regular install, then give the computer
- When gnome-initial-setup was first introduced years ago, I didn't like the idea of splitting the installer questions in two. Maybe I was thinking of installers like Windows where even after answering the second half of questions, it would take a long time before the computer is ready. gnome-initial-setup does not take long at all before your computer is ready to go.

Disadvantages
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There isn't a way to set up encrypted home directories with ecryptfs.