Update theme for Ghost v4

[backspace]

These are some minor changes to prepare for migrating to Ghost v4.

The significant one is changing the height of images in articles to auto, as suggested in the migration guide.

When I do that locally, before and after:

This is what it looks like on v4 without this change:

It seems preferable to me to show the full height of the image 🤔

Something unfortunate is that the Heroku-adapted version of Ghost used for preview deployments isn’t yet updated for v4, but hopefully that can be updated soon.

[farfamahar]

Looks great. Full height definitely looks better 👍