8014, 8059: Unify Rails package building.

8014, 8059: Unify Rails package building.

* Introduce a handle_rails_package function, and helpers, to build a
  Rails package.
* Generalize all the support scripts into unified postinst, prerm, and
  postrm scripts.  handle_rails_package builds these into the
  packages.  See jenkins/rails-package-scripts/README.md for details.
* More error checking throughout (e.g., check for failure when doing
  any prep work for Rails packages, or the Workbench packages
  specifically, using `set -e` in a subshell).

One behavior change from this unification: before this,
run-build-packages treated --build-bundle-packages to mean "*also*
build bundle packages", while run-build-packages-sso treated the
switch to mean "*only* build bundle packages".  This commit declares
run-build-packages to be the winner, and makes run-build-packages-sso
consistent with it.