This time, add it as a wrap module rather than a git submodule. They’re
easier to manage, and integrate better with Meson.

The subproject has to be re-added so that malcontent can be built on
Debian Stable and Fedora 31 for the gnome-software CI. See
https://gitlab.gnome.org/GNOME/gnome-software/-/merge_requests/487.

Signed-off-by: Philip Withnall <withnall@endlessm.com>