Split up tor-browser-build into a generic project and tor-browser related parts

Over the weekend I wanted to look at relay-search and how I can modify code in it. I had some trouble compiling it and ended up working on a reproducible builds environment for our metrics projects. I heard @richard and @JeremyRand started from forking off of tor-browser-build and it was... not an optimal experience. I started from the other side: a project just with rbm as submodule, picking from tor-browser-build what I actually needed. While doing so it felt to me that all the consumers of our reproducible build system would benefit if we could move the shared tor-browser-build items in a separate project. That might help the adoption of our reproducible build system, too. What I thought worth sharing and maintaining in an own project was:

• the container creation and handling
• the infrastructure dealing with git/mercurial repos, signed tags etc.
• all the reproducibility related bits
• dealing with old artifacts (that is make clean things)
• a default example rbm config file dealing with the minimum of config options (like number of CPUs, or logging location etc.)

/cc @boklm, @richard, @JeremyRand