Currently we use handlebars to output raw data from:

• {{image}}
• {{author.image}}

These output relative url paths to the image files. It would be helpful if these were turned into helpers supporting an absolute value, eg.

• {{image absolute="true"}}
• {{author.image absolute="true"}}

With output including @blog.url prepended to the image path.

Primary use-case: For valid output in meta tags, eg. in #3900