Add outgoing email testing from the UI

[Trapulo]

Is your feature request related to a problem? Please describe.
A function to test email delivery from the GUI will be useful to check the environment.

Describe the solution you'd like
In admin area a form where to insert an email address and a button to send a test email

[Bubka]

I'm working on a dedicated admin section, this will fit in perfectly. Thanks for the suggestion 👍🏻