Add Form to send test email

Problem/Motivation

The settings form is also the test form. When sending a test email there is a concern that the settings could be updated. Either accidently by user error, or by brower auto completes.

Sending a test email should not update any configs.

Steps to reproduce

Proposed resolution

Create a seperate form for sending test emails.

If reroute email address is set, the test email field is disabled and contains the reroute email address.

If reroute is not set, the test email field is prefilled with the current users email address.

Create a ConfigOverride class. We want to override the system.mail default.interface to use SMTP, but only for the test email. We want to limit the override to the request.

Remaining tasks

User interface changes

Add a 'Test' tab next to the settings.
Add a form to send a test email.
Remove smtp_test_address from settings form.

API changes

Data model changes

Remove smtp_test_address from smtp.settings. It is not used. The existing form shows a blank string, and the form does not save the value.