Add summary.Visual image options

[shapiromatron]

Add image options to summary visuals, after noticing some high resolution pixels scale and look way too big for a screen.

Added two new options when configuring a summary image:

• alt-text (required): text required to display on image for accessibility purposes
• max_width (optional; default 1000px) - if specified, the maximum width of an image. If unspecified, it will grow to 100% of the image width in pixels. The image now auto-shrinks on mobile views too.

Example; max width of 600px:

And a mobile view of 400px wide:

[munnsmunns]

Everything looks good, it just needed formatting for the HTML changes