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.
Refactor Invoice#copy by moving logic from controller to model.
Guess period when copying invoice.
Couple of gem updates.
Update rubyzip and selenium-webdriver.
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.
Merge branch 'features/backup_with_schema'
Update CHANGELOG.
Drop delete links in backup list for now.
Add icon to backup button in tenant view.
Make #interpolation_options more robust to not crash application if flash message cannot be built.
Change backup creation to use #dump instead of #create.
Support importing backup files.
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.