Downgrade Sharp to 0.17

WebP images are broken (in some cases) with > 0.17, see lovell/sharp#1363, lovell/sharp#1242 and #64

It's not totally known if this might impact the smart cropping, but given that we need webp working more than smart cropping (as webp came first, we shouldn't break existing features) I'm willing to take that risk (smartcrop I think shows 0.20 as a peer dep).