Support GVFS admin:// backend

It gets harder and harder to use old fashioned ways of escalating privileges, such as sudo and gksu, as distributions continuously curtail them over both real and esoteric security concerns.

Root use of [dbus](https://aur.archlinux.org/packages/dbus-x11/#pinned-682768), for example, is generally frowned upon although there are hardly any alternatives.

GVFS offers a [backend](https://wiki.gnome.org/Projects/gvfs/backends) that can work around having to use sudo for dbus and other applications, but code doesn't seem to understand it. Trying to open a file with the [admin](https://wiki.archlinux.org/index.php/Running_GUI_applications_as_root#GVFS) backend, such as ```io.elementary.code admin:///etc/hosts``` results in:
```
[FATAL 12:13:43.478153] [GLib] g_file_set_contents: assertion 'filename != NULL' failed
[WARNING 12:13:43.478603] Application.vala:183: The specified location is not mounted
```
Code then opens with ordinary user privileges.

If the backend is supported, the expected result would be a prompt for administrative password and code opening (or at least opening a tab for the specified file) with administrative privileges.