[Feature] Calendar should offer reminder notifications

[rapenne-s]

It would be really useful if Peergos calendar was able to send reminder notifications through the web browser. This mean the calendar app should be updated to ask for setting the reminder(s) (could be one or many).

Ideally, this could be per entry, but a first implementation could have a global reminder defined and configurable?

For instance, there are weekly entries I would like to me reminded about 1h before and 2 minutes before, while some entries are just there for information only and I do not need to be reminded about them.