Make 48h meeting reminder customizable

[paarals]

Is your feature request related to a problem? Please describe.
An automatic reminder is sent to registered participants of a meeting 48h before the meeting begins. In our practical experience, this is not ideal and lacks two degrees of customisation:

1. I might want to add additional context to the reminder (updated meeting information, additional details only for registered participants)
2. 48h before the meeting might not always be ideal. For some meetings, (i.e. such with a longer registration period), I would like to have a longer period, for other meetings, I might not want a reminder at all.

Describe the solution you'd like
Make reminder e-mail/notification customizable, namely with:

1. Checkbox, whether it should be sent at all
2. If yes, having the possibility to adjust the time of sending (in h) before the start of the event
3. Make the content customizable via text editor (as is already possible with invitation emails)

Tasks:

• Add configuration options in the admin interface
• Add configuration options in the public interface (user created meetings)
• Configure the reminder meeting task to use the configured time and the customized message (if any). If no message is specified defaults to the current one