[NEW FEAT] Allow for scheduled sending of drafts created by this add-on

[ttsukagoshi]

Is your feature request related to a problem? Please describe.
In cases where you'd like to schedule an email to a group of recipients, it would be nice to have this feature incorporated in this add-on so that you can set the timer for sending the merged mails at once.

Describe the solution you'd like
Create a button next to the Send Created Drafts button: Schedule Sending of the Created Drafts

Describe alternatives you've considered
A workaround currently available is to set the schedule on per email basis. That is, if your recipient list isn't too long.