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Improve the directory migration

Previously we had:

previous/
  <date-time>/
    db_dump.sql
    dev/
      apache/
      deploy/
      django/
      ...

But after, for consistency, we want to have:
 <date-time>/
  db_dump.sql
  apache/
  deploy/
  django/
  ...
Remove old create copy for rollback function

from fablib - no longer required
Change ordering of when to do database dump

Might be root cause of database dumps not happening
Compress the db_dump for rollback
Correct path of old rollback directories to delete
Purge prev_root from fablib

Only need it when migrating directory structure, so create and
use it there.  Any other usage of it will give an error.
list_versions() should only list timestamp directories

Also show the current version, and rename from list_previous()
fix minor bug in _migrate_directory_structure()
rename dump db function to make more sense
Refactor how we get list of versions

Put it in a function to do it well, and use it everywhere
Update rollback version to use new directory structure

Simpler in many ways