render markdown body only on request

[karthik2804]

This PR adds a new handlebars helper render_markdown.

This helper allows us to conditionally render the markdown in the body which helps with rendering performance as we currently render the entire set of pages when indexed on each rendering.

This would be a breaking change wherein occurrences of

{{{ page.body }}}

would need to be replaced by

{{{ render_markdown page }}}

[tschneidereit]

Looks good to me, but I would prefer someone with more knowledge of the system sign off on this