Allow path outside site files directory for images

Problem/Motivation

We have many sites that we'd like all to use the same branding logo and wallpaper images, as they belong to the same organization. Instead of uploading these images for every site, it'd be simpler if we could simply point them all to the same path, e.g., /themes/custom/login-logo.png and /themes/custom/login-wallpaper.jpg -- not only does this allow us to change the images for all of them at once if we desire, but it allows for an easily portable configuration or recipe.

Currently, trying to set the path to anything outside the site's own /sites/foo/files directory results in an error.

Proposed resolution

Not sure if the best way to handle this is to loosen up the fields' validation rules, or to add an option for a direct image path that hides the upload field and changes validation on the path field.

Remaining tasks

User interface changes

API changes

Data model changes