Events (how to attach date-related data to any item?)

_Many_ items on several kinds of list can have date-time related data. e.g:
+ a task that has a deadline. `SELECT * FROM events WHERE item_id=123`
+ a checklist for a [morning routine](https://github.com/dwyl/product-roadmap/pull/11#issuecomment-488224182) that reoccurs once each week day.
+ a reminder to [send a birthday card](https://github.com/iteles/All-the-Hellos) 3 days before a person's birthday. 😉

The following is the _outline_ of the schema I have in mind: (_definitely not complete!_)

+ `event` - when an item has dates & times associated.
+ `id`: `Int`
+ `item_id`: `Int` (**FK** `item.id`)
+ `kind`: `Int` (**FK** `event_kind.id`)
+ `start`: `NaiveDateTime`
+ `end`: `NaiveDateTime`
+ `frequency`: `Int` (**FK** `event_frequency.id`)

+ `event_kind`
+ `name`: `String`
+ "deadline"
+ "meeting"
+ "reminder"
+ "class"

+ `event_frequency`
+ `id`: `Int`
+ `description`: `String`
+ "once" - if an event is a one-off and does not repeat.
+ "hourly" -
+ "every-waking-hour" - e.g: "stand up from your desk every hour"
+ "morning-and-night"
+ "daily"
+ "week-days"
+ "weekends"
+ "weekly"
+ "monthly"
+ "quarterly"
+ "annually"
+ "custom"
+ "First Tuesday of the month"

+ `event_invitees` the `people` who are invited/required to attend an `event`

Contributions/discussion very much wanted/encouraged!