Force re-send all emails regarding the schedule

## Problem you are facing

Same problem as when I accepted talks, I wanted to change the template so I discarded all the emails in the outbox, and now the only way to re-send all the emails is to move around all the talks. 

## Possible Solution

Is there a table I can update in the database to make pretalx forget that is already sent the initial email?

## Context

I'm on v0.7.1.