Automatic email reminder sent to participants of an event e.g. 24 h before event

[ddabble]

[Originally created on the "Dev-Web" Trello board, by @shahzebs on November 21, 2019]

This might increase attendance.

Users need to consent to this usage of their email address (due to GDPR), so we need to add some kind of consent form before we can do this.

[ddabble]

This is partly resolved by #378 (missing scheduling sending the emails, and adding the consent form mentioned in the description).