'goal builddict' : generate .rst and .html, not just .rst

before: running 'goal builddict' generates a couple of .rst files which we then give to Sphinx

after: running 'goal builddict' generates those .rst files but also a couple of .html files which we then give to sitegen.

If/when sitegen's in good shape, it'd be good to get rid of the .rst-generating part.

Most of this change is in src/python/pants/backend/core/tasks/builddictionary.py , plus new html templates.

There are also changes to some docstrings:

I replaced some sphinx-rst-isms with docutil-rst-isms (which also work with Sphinx).

And I replaced some lost docstrings which I removed with the "inherit docstrings" change because I "knew" they'd be inherited from a superclass.

And added some docstrings which have been missing for a while which I noticed because increased scrutiny grrrrr.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/36888156

Running sphinx on the generated .rst files now generates warnings :-(
I poked around 'em for a while, but eventually gave up.

Bugs closed: 529

Reviewed at https://rbcommons.com/s/twitter/r/1079/
Sync of https://github.com/pantsbuild/pants/commit/32bde41ddac614788113f6d1482a8ca81930a080