Media/attachment library

[streamich]

Add ability to store media/attachments (such as images, files) in the document. This is useful in a setting where the app that embeds the editor does not have an external media storage facility.

• Ability to store media in a document.
• Ability to reference media, multiple references.
• Render media, all attachments, screen.
• Allow to opt out of media/attachemtns.
• Ability to provide custom media storage.
• Ability to specify max media size, as a function, could depend on media mime/type.
• Use con(bin()) for storing large binary data.
• For images, for large ones, support resizing to a smaller image.
  • Draw image on <canvas> and convert back to PNG. See browser-image-resize.