[13.0] [IMP] database_cleanup: purge orphaned SQL views

[Linekio]

Improve the "Purge obsolete tables" feature to purge SQL views. SQL views are not automatically dropped after uninstalling an addons, or upgrading odoo. For example, the view mail_statistics_report is not dropped after migration to v13: https://github.com/odoo/odoo/blob/12.0/addons/mass_mailing/models/mass_mailing_report.py#L32