[feature] Calendar and icalendar meetings

[donrestarone]

Calendar and Meetings

addresses: #1597
This release adds the ability for Violet to handle your calendar. Incoming emails with .ics attachments will automatically be added to your calendar as meetings.

Calendar UI

Outgoing Meeting request RSVP controls

todo:

1. include .vcs file for outlook
2. add validations to meeting model

further reading:

1. icalendar syncing events: https://joshfrankel.me/blog/lemme-pencil-you-in-using-icalendar-and-rails-to-sync-calendar-events/
2. all the options: https://blog.corsego.com/icalendar-ruby
3. publish? https://stackoverflow.com/questions/55927263/what-does-icalendar-publish-method-do
4. dealing with email client quirkyness when displaying RSVP buttons: https://stackoverflow.com/questions/66102584/when-i-add-method-request-to-icalendar-gmail-stops-recognizing-as-event