Resize large showcase images

[jonoaldersonwp]

The showcase image at ​https://wordpress.org/showcase/blondie/ is 3.5mb.

​https://wordpress.org/showcase/thisisfinland/ is similarly enormous.

There's no way that these images ever need to be bigger than 50kb. Could we bulk-optimize these down to sensible sizes?

[dd32]

These images are supposed to be served through photon, as evidenced by the ?w= suffix to the image urls.

I'm not sure why it's not enabled, and I can't seem to find the option to enable it anymore.

[dd32]

In 10664:

Showcase: Use Jetpack Photon for uploaded images, this adds the support for resizing via the w arg.

Fixes #5612.