Add post content to RSS feed

[jtdaugherty]

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At present the RSS feed includes only the page title because the page content uses relative URLs to reference generated images (such as LaTeX math), which won't work in an RSS feed. However, generating content with absolute URLs is unfriendly for blog testing where one wants to run a local instance without having to worry about the particular production URL that the blog will eventually use.

It's possible to have mb use a different config for 'production' or 'development', but it isn't possible to force absolute URLs on all page resources.

So: we need a way to force absolute URLs on everything (i.e., generated images, next/prev post links, the post index, stylesheets, etc., etc.).