Slack endpoint needs fallback text

[jloh]

The slack endpoint currently doesn't have any fallback text. This is very poor form and goes against their standards!

geojs/conf/v1/hooks.conf

Lines 255 to 268 in c7e8c5b

	message = {
	response_type = slack_response_type,
	attachments = {{
	mrkdwn_in = {
	"text",
	"pretext"
	},
	fallback = "", -- TODO: Actually put something here good lord
	pretext = title,
	text = resp,
	footer = "Powered by <https://geojs.io|GeoJS>",
	footer_icon = "https://static.jloh.co/geojs/avatar/hipchat/v1/geojs-icon_2x.png"
	}}
	}