Add continuous deployment script. No self-updating of the script itself yet.
Decouple and stop hard-coding things.

* Lift up _latest_successful_build() to make testing easier.
* Allow command-line customization of just about everything.
* Take the newline-stripping out of run(). It turned out to be seldom useful--not worth the coupling.
This is about 200 chars (but only 1 line) shorter and has less "self." noise than the class-based version.
Fix bugs so the thing actually runs. Change some confusing vocab.
For now, let old versions of the deploy script deploy new versions of the code.

It may well never be worth the time, complexity, and effort to do better.

So we merge the deployment-only requirements file with the one for DXR proper, and we'll run the deployment script out of the deployed DXR venv.
Add continuous deployment support.