Cleanup by Time

Hi zimbo,

the stepper has a granularity of 10, with min of 1.
Agree, integral multiples of 7 would be an even more common value.
Using the typed in custom number would be even better, but is not supported by the settings framework I’m using without further adaptation (maybe with a compiler hook).

I might change the value to 7 in the next release at least.
https://github.com/No3x/wp-mail-logging/issues/26