Do not promote or show classic snaps as installable or display the warning before installation

[Jean-Baptiste Lallement]

artful desktop

Test Case:
1. Search atom (or any classic snap) and install it

Actual result
It is not installable in gnome-software and fails without a user visible error message.

Expected result
When a classic snap is installed from the command line, there is a warning message that explains the risk and asks the user to repeat the installation with --classic if he understood the message.

In gnome-software it just fails without any message. There are several options:
1. Do not show classic snaps
2. Show the warning and the user must acknowledge before proceeding with the installation, in which case the snap is installed in classic mode.

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: gnome-software 3.24.3-0ubuntu6
ProcVersionSignature: Ubuntu 4.11.0-10.15-generic 4.11.8
Uname: Linux 4.11.0-10-generic x86_64
ApportVersion: 2.20.5-0ubuntu5
Architecture: amd64
CurrentDesktop: GNOME
Date: Tue Jul 18 14:02:36 2017
InstallationDate: Installed on 2013-09-03 (1413 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Alpha amd64 (20130902)
InstalledPlugins:
 gnome-software-plugin-flatpak N/A
 gnome-software-plugin-limba N/A
 gnome-software-plugin-snap 3.24.3-0ubuntu6
ProcEnviron:
 TERM=screen-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-software
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Robert Ancell (robert-ancell)]

Bug 1690280 is for adding support for classic snaps.