Fix symlink handling in content directory.

In preparation to use git-annex for tracking large binary files, we
need to solve some problems related to its symlinks.  Most importantly
is that the nanoc "filesystem" data source really doesn't like broken
symlinks, so we modify that to avoid major problems.

Next we filter out all missing annex items in a preprocess rule.
Ideally we would replace them with stubs but just dropping them is
acceptable for now.

Finally, for binary items, passthrough rules end up copying the symlink
literally from the content to the output directory.  In the case of symlinks
to the annex directory this is not going to deploy correctly, so we need
a dummy filter to break the link.