Add a transitional package which hopefully does the upgrade in the
right way (following https://wiki.debian.org/RenamingPackages
except that adding the transitional package to the new package's
control file make a package that reprepro rejects...)
The transitional packages can be built with:
```
dpkg -b riot-desktop_1.7.0_all
```
The nightly transitional package is named / versioned assuming a
release on Monday the 13th and will need to be edited & renamed
as approriate (it just needs to be greater than whatever the latest
nightly is at that point).
