Support multiple calendars

[phansch]

Currently events are independent of each other and don't belong to a specific calendar.
As discussed in #2, in the initial version, a user should be able to:

• Add a new calendar (New users should start with an example calendar to start off with)
• Add events to the calendar
• Remove events from the calendar
• Remove the calendar, including all events
• Share the link to the calendar

[carolinagc]

Here is a diagram of the models.