Implement basic backup/restore.

This feature has been backported from master. It includes adding the
yaml_db gem which is wrongly in #52229b87 on master. It also includes the
tenants/form refactoring from #b8e3273e which refactored for the setup
wizard.
Update CHANGELOG.
Update rubyzip.
Implement support for upgrades from older backups.

This is done by using a zip file as backup. This includes both the YAML
data and a schema.rb. When restoring, we create an empty database using
the schema.rb, import the data, and then run all the pending migrations.

Conflicts:
	app/models/backup.rb
Drop delete links in backup list for now.
Add icon to backup button in tenant view.
Change backup creation to use #dump instead of #create.
Support importing backup files.
Add rubyzip to Gemfile and require zip in Backup model.
Some commenting fixes in Backup.

Conflicts:
	app/models/backup.rb
Some commenting fixes in Backup.

Conflicts:
	app/models/backup.rb
Refactor Backup#dump into multiple methods.
Refactor Backup#import into multiple methods.

Conflicts:
	app/models/backup.rb
Better class docs for Backup model.
Rename Backup#import to #restore.

Conflicts:
	app/models/backup.rb
Restructure Backup.
Drop Tenant#export.
Refactor Backup method so it has also a class method interface for command line.
Add UPGRADE.textile.
Use bundle exec rails runner in restore step in UPGRADE docs.