The Sphinx 2.0 release contained a few incompatible API changes that broke

our extensions and, thus, the documentation build in general.  Who knew
that those deprecation warnings it was outputting actually meant we should
change something?  This set of fixes makes the build work again with
Sphinx 2.0 and eliminates the warnings for 1.8.  As part of that, we also
need a few fixes to the docs for places where the new Sphinx is more
strict.

It is a bit late in the cycle for this kind of change, but it does fix
problems that people are experiencing now.

There has been some talk of raising the minimum version of Sphinx we
support.  I don't want to do that abruptly, though, so these changes add
some glue to continue to support versions back to 1.3.  We will be adding
some infrastructure soon to nudge users of old versions forward, with the
idea of maybe increasing our minimum version (and removing this glue)
sometime in the future.
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