Modern versions of docutils and Sphinx allow for setting certain kinds

of page metadata, using .. meta::. This is useful for SEO/social media

page descriptions, for instance.
The problem is, these get compiled directly to a block of <meta> tags,

which isn't what we want in our case, since we render docs more

explicitly and need to use this metadata for multiple things.
This change introduces an extension that walks the document, extracts

the metadata, and stores it in the document's structured metadata

storage, where we can get access to it.

Unit tests pass.
Tested this with the Review Board documentation, verifying that data was

stored and could be used.