[Feature: Both versions] Add charts / statistics for sent emails

[Lednerb]

Would be nice to have also some information in the dashboard about the sent emails per day.

Maybe you can integrate this information to the "Delivered emails" chart?

[SH]

Thanks!

We will rewrite whole dashboard since it is slow on large installations. We will also create more informative statistics.

[Lednerb]

:+1:

[SH]

Done.