Improve soledad debian packaging

Some things we want from the packaging process:

• build in clean chroots (git-pbuilder can help there)
• allow for easy update based on upstream code (git-buildpackage can help there)
• build for all the architectures/distributions we support (amd64, i386; sid, stretch, jessie, vivid, wily, xenial, yakkety)
• allow for easy creation of packages based on feature/bug branches
• move to non-native package format

(from redmine: created on 2016-11-24)