Add a notification for events taking place soon

[AlbertoPdRF]

Is your feature request related to a problem? Please describe.

Users sign up to attend events, but they are never reminded about the event taking place when the time of the event approaches.

Describe the solution you'd like

It would be awesome to generate notifications for users attending an event when such event gets close enough in time (e.g. less than 24 hours). For that, I guess the best solution is to set up a cron job on the back end that checks for events happening before the agreed-on close enough time and generating the corresponding notifications.

Describe alternatives you've considered

Keep everything as is.

Additional context

This came up originally in #26, but I'm extracting it from it. Also, I've been discussing this with @san-oop on Slack, and he might be interested in trying to take care of it! 😃