[MlMt] Show "send later option always

[Andreas Jung]

I moved my Mailmate installation and I can’t no longer see the Send 
later option inside the composer window.
The enabled both related hidden preferences and the Send Later option 
show up again with the default value.
However I want the default to be 0 seconds and I want that the Send 
Later option field is always available
within the composer window (as it was on my old system).

-aj