At the moment, if the user attempts to install a snap that requires classic confinement, the install fails.

For the command line UI, the user has to pass the extra "--classic" argument to install such snaps, which is converted to a flag on the install API call.  This is intended to get the user to acknowledge that they are installing third party software that is not sandboxed from the rest of the system.

The first patch is an updated version of Robert's that simply allows installing of classic confinement snaps, which I think is appropriate for upstream use.

The second patch adds a warning to the package details page when displaying such snaps, which might be an Ubuntu only change.  I realise that this doubles up with the existing "Sandboxed" kudo badge, but I do think we need a bit more than just that badge.  At present, there are two reasons why that badge might not display on a package on Ubuntu:

1. we're showing a .deb package from the Ubuntu archive, managed by a trusted set of developers.

2. we're showing a .snap package from a third party developer that requires classic confinement.

(1) is going to be the most common case, and is generally not a problem, so I suspect few users will appreciate the lack of the badge in the case of (2).