Initial support for online events

[cbellone]

Given the current restrictions due to the pandemic, it is not advisable nor permitted to organize in-person gatherings.
We must therefore modify alf.io to support online events. Some notable features we're going to implement:

• support one or more "links" to online meeting tools, or calls, for the whole event or for a specific category (category setting overrides event's)
• do not send Ticket PDF or Wallet Pass, as they are not meant for online events
• include a "check-in" URL in the confirmation email. By clicking on the confirmation URL, at the proper time, the attendee will be redirected to the online call