Add new feature to link the bot to a Google calendar and show event info

[vikvanderlinden]

This feature would have similar features to e.g. ChronicleBot but be much more customizable.
These features are:

• Detect new events in the linked calendar
• Detect updates in the linked calendar
• Post messages to a specified channel about the calendar events (here, customization is very interesting, to add description and location etc)
• Add a Discord event for each calendar event and keep it up to date