Demystify make targets and deb packaging

Changes for the make build setup:

• simplify handling of virtualenv (created automatically, if not disabled)
• cooperate with makefilet (extend instead of overwrite targets)
• avoid recursive make execution (emitting warnings for target overwrites)
• use stamp files in order to avoid inconsistent states
• remove app_setup target (duplicate of app_run)

Changes for the deb packaging:

• split into multiple binary packages (code, data and dependencies)
• rename from stadtgestalten to grouprise
  • requires adjustments (rename of directories and username) during the upgrade of an installation
• use make-based packaging instead of dh-virtualenv
  • dh-virtualenv required a lot of complication, since our setup is not completely covered via setup.py
• remove all arch dependent components
  • now the package can be built on any architecture
• remove interference of system-wide installed packages
  • previously the set of locally installed packages influenced the packaging result

The documentation was updated to the current state of the build procedure.

The packages installs successfully in a clean system. The setup works without further adjustments after enabling UWSGI and nginx. Thus I assume, that all assets are properly included.