Finish Implementing Auto Cross-References

This is the final change to implement autocrossref. Basically, it gets us back to where we were a week ago (!) when we applied the manual crossrefs, but with all the benefits of autolinking.

My preview site: <http://test.dtrt.org/en/developer-guide>

This commit changes more lines than GitHub will show---and way more than you'd ever want to read, so I massaged the git diff output a bit so you can actually review the changes:

* HTML: <http://dtrt.org/tmp/autoxref.diff.html>

* In your terminal: `git diff --word-diff=color -U0 | sed '/@@ /d; s/^[^[:cntrl:]]*\([[:cntrl:]].*[[:cntrl:]]..\)[^[:cntrl:]]*$/\1/' | less -R`

Here's a summary of what I changed:

* This change only removes brackets (and, in one case, adds two brackets). No other changes were made to the dev guide.

* A few terms I missed previously were added to _autocross.yaml.

* Links checked; all internal links link to valid ids except the link definitions including "FIXME". (See bottom of the file.)

* A few links were left in where they linked to terms I didn't think should be autocrossref'd

In the absence of critical feedback, I will merge this request in 24 hours (18:00 UTC on April 15th).