[concept: Events] Create events to broadcast to site users

[sreecodeslayer]

Each site can accept events from Nudge. An event can be considered as a message or a promotional campaign which the site maintainer wants to publish to its users. To do this, we will broadcast the event via the connected site channel.

An event can have the following properties:

• kind: for now this will be "banner" at top. we will support more types in future
• title: emphasized word
• body: string (or html?)
• cta: a button link to which users should be redirected on click
• start_datetime: utc
• end_datetime: utc
• belongs_to Site
• respect_dnd: boolean [if enabled, we won't show event to user]

Sub tasks

• [Events] Context with schema for CRUD #9
• Broadcast event when its time to subscribed users #10
• Push event to new users that join the site when event is ongoing #11
• Create events via API if authenticated #12