Intercept (and upload) pasted images

[michael]

Currently when you paste images into the editor they'll be converted to base64. But we don't want to store base64 strings in the HTML in our database.

We should be able to handle:

• Direct image pastes (like copy and paste an image from the filesystem)
• Implicit image pasts (you paste a snippet of HTML which contains images)

In both cases we want to upload the images to MinIO, so everything is properly self-contained.

[michael]

Implemented in PostOwl, we just need to carry this over here. We can think about handling cmd+p paste (when there is only one image, and no HTML in the clipboard).