Added the migrations for migrating a GN 1.2 instance to GN 2.0

Added the migrations for migrating a GN 1.2 instance to GN 2.0 instance.
Documentation is included.
We have been using them successfully to migrate our instance here at WFP.
We are still doing some testings but everything looks working so far.
I believe the installation procedure will need to fake all of the migrations (but maybe is already the case).
I would strongly recommend to remove all of the migrations stuff and start from scratch when beginning development for 2.1.