implement calendar functionality

[donrestarone]

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Is your feature request related to a problem? Please describe. currently there is no calendar feature

Describe the solution you'd like

1. ability to show a calendar
2. ability to pick working hours
3. book events (unconfirmed, confirmed, maybe, declined) and send invites via email
4. parse .ics files in incoming emails and create event
5. ability to send a link to a booking page

Libraries:

1. https://github.com/icalendar/icalendar - PORO for calendar, event and includes parsing functionality
2. https://github.com/excid3/simple_calendar - frontend for rendering a calendar

research:

1. simple_calendar example implementation: https://medium.com/@zachmb22/how-to-display-calendar-events-using-a-rails-gem-6db0b1e74f62
2. using stimulus JS for a calendar implementation: https://dev.to/timnans/rails-calendar-with-tui-and-stimulusjs-3je