Add route to persist history.yml on demand

[andyhayes]

Workaround for issue #244 (history.yml file not being written when server not shut down cleanly) but probably useful in other scenarios too.

Adds post /system/persist-history route to persist the history.yml file to disk on demand. As with other routes, it requires a json body with valid auth_token.

Not 100% sure on the route path, so happy to change if anyone has a better suggestion.

[jbfarez]

This sounds really really really GREAT !

[j1n6]

This sounds like a great idea, I will add this API to dashing-contrib

[terraboops]

This is a cool idea, though it might be nicer to simply persist the history whenever send_event is triggered -- debounced so that it is only written once every minute or so. Somewhere in here maybe:

dashing/lib/dashing/app.rb

Lines 133 to 139 in 439182f

	def send_event(id, body, target=nil)
	body[:id] = id
	body[:updatedAt] ||= Time.now.to_i
	event = format_event(body.to_json, target)
	Sinatra::Application.settings.history[id] = event unless target == 'dashboards'
	Sinatra::Application.settings.connections.each { |out| out << event }
	end

This depends on how expensive it is to write the history to disk...