Event post type

[paulrobertlloyd]

• Add (optional) location fields with localised strings
• Add start and (optional) end datetime fields with localised strings
• Show location and start/end datetime fields for event post type
• Add validation for name and start date for event post type
• Add event post type to list of supported post types
• Add <event-duration> element that allows toggling between input[type=datetime-local] and input[date] depending on whether times are needed for the event
• Update deriving location property so location can contain a venue (h-card) or address (h-adr) and/or geographic coordinates (h-geo)

In future iterations we can use geolocation to find an address and geographic coordinates from a fuzzy free-text search, but this simpler implementation means Indiekit can support creating this post type sooner.

Fixes #541