download: Note Ubuntu PPA as Unofficial

[wbnns]

This fixes #1506 and provides additional clarity to the Ubuntu link by
adding a note that the PPA is Unofficial:

Unless others object, this will be merged on Wednesday, February 15th.

cc: @harding @Pixdigit

[harding]

Perhaps Note: unofficial should be a translated string? Otherwise LGTM. Thanks!

[luke-jr]

In what sense is the PPA unofficial?

[wbnns]

@luke-jr Heya Luke, here's the context with regard to the issue that was opened - #1506.

[luke-jr]

I consider the PPA official and recommended for Ubuntu users. It's technically less secure in a sense, but in the same way that using Ubuntu (or any OS) implies trusting the vendor maintaining it.

[wbnns]

Let's put this on hold to see if anyone else wants to share feedback - also, FWIW, perhaps leaving it as-is, unchanged, could be a good option.