Improve NNexus UI and other integration aspects

[holtzermann17]

We now have a preliminary working correction with the autolinker, but it only does linking, it doesn't yet update the index, and the user has no way to control the link policy. There are also some minor XML bugs.

• the autolinker should be particularly useful in the Problems section of the site (our basic use case: someone uploads a problem precisely because they don't know what the terms mean, the autolinker then helps them get the background material).
• When articles are deleted, we need to clear any relevant LINKING caches and update articles that link to the now-deleted article.
• If NNexus adds RDFa triples that say "This text links to Article B" (for each auto-linked article B), then I ought to be able to do a very nice trick in Drupal, namely, expanding a given text to show its "context". This would be particularly useful for problems, since we could pull in documents that define the terms used in the problems.

[holtzermann17]

See also:

• https://github.com/dginev/nnexus/issues/4 (RDFa in links for NNexus)
• https://github.com/dginev/nnexus/issues/3#issuecomment-12293077 (using a backlink list to un-link in the case of deletion)

[holtzermann17]

NNexus improvements will be meaningful for the editorial workflows we want to set up on PlanetMath. See https://github.com/holtzermann17/planetmath-docs/issues/10