Clean up Form Package

This PR cleans up the Form package to remove some CMS remnants.

- Remove JFactory from Form class.
- Remove JFactory from Rule\Email. Remove unique check, because it's for the CMS.
- Removing SQL field type. Create a custom field type instead.
- Remove Factory from Field\Timezone. Fix usage of JHtml.
- Remove unused tests.
- Remove ComboBox field type. Not used anywhere (not even in the CMS) and requires media.
- Remove password strength meter. Requires media.
- More cleanup

Thanks @mbabker for helping answer some questions on this. :+1: