Let page authors set default captions for images

[radiatoryang]

When reusing an image throughout multiple pages, it is annoying and error-prone to write the same text caption (e.g. image description, attribution, or CC license info etc) over and over for every use on every page.

Ideally, the caption could optionally be stored on the image object itself. This would make it easier to sync and maintain the image caption across multiple pages. And when you add the image to a page, it would auto-populate its own caption text.