Customizable site header and footer

[booski]

The site header should be customizable, along with the footer.

Since at least the header can include an image (or as right now, two), some kind of handling of that will be necessary. Base64 embedding of images might be the best option here, since it makes it possible to include an arbitrary number of images in the header without worrying about storing uploaded files. It would need to be implemented such that the user doesn't need to do the encoding/embedding though.

One option to implement image embedding would be to use https://github.com/arcs-/medium-button to create a custom button in medium-editor that would wrap the upload->base64->embed flow.