Repeated reminders for Draft proposals post CFP close and other checks

[neomatrix369]

• Various checks to prevent the scheduler from sending out a reminder when the CFP date has closed and when its nearing the schedule closing date.
• Also preventing the button in the admin page to send a reminder when CFP has already closed. There is still a discussable part i.e. where Akka.system.scheduler.schedule(initialDelay, everyX days, ZapActor.actor, DraftReminder()) is used which can set reminders forever.
• Minor date formatting in a few email and show proposal templates.

Akka Scheduler docs: https://doc.akka.io/docs/akka/current/scheduler.html

Trello cards:

• https://trello.com/c/J4YTZJCT/32-cron-job-is-sending-reminders-about-pending-drafts-not-submitted-past-cfp-close-date
• https://trello.com/c/regNKGGj/76-speaker-is-receiving-reminders-about-delete-draft-proposals