Fixes #17844 - unify test mail js code

Fixes #12156 - Adds email configuration into Foreman settings

Instead using email.yaml, the email configuration
can be modified dynamically via settings.

Fixes #17120 - Allow custom name for setting category

Fixes #12945 - use helper for table css classes

Fixes #7167 - names of settings marked for translation

Fixes #9589 - Add documentation button to various models

Fixes #8785 - HTML syntax fixes - add thead/tbody, fix formatting

fixes #2407 - Add STI to settings model

Also adds some tests for rendering new/missing STI models, and
refactors the old default_settings loader into the new sub-models.

Feature #2368 - form helpers, exceptions and fixes

Feature #2368 - host list popups and various fixes