[#5973] rename rebuild.bash to rebuild-all.bash with more helpfulness
[#5973] Make the pysvn dep explicit, so we fail sooner (setup.py develop instead of runtime import)
[#5973] remove ancient README within the Allura dir since we have a top-level one
[#5973] Simplify the README

* explain SVN is optional (often tricky to install on some platforms)
* use rebuild-all.bash
* omit SCM symlinks since they don't add anything useful (/tmp works just fine for getting started)
[#5973] update repo location
[#5973] move ipython requirement since it is used in tests
[#5973] symlink pysvn instead of using --system-site-packages
[#5973] update vagrant scripts

* update instructions
* update ubuntu base
* various other fixes & improvements
[#5973] mention port forwarding
[#5973] remove sf.net-specific shorturl migration