Fix Slack social media icon

[4mber]

Currently clicking the Slack social media icon on a users info window only opens Slack, rather than opening a message with them.

I believe this can be accomplished by using Slack API's users.list method within the map.on('click', function), then checking if a member's SlackID (feature.properties.SlackID) is listed, and if so returning the ID. The ID would then be added into myHTML to complete the deep link (slack://user?team=T6963A864&id=xxxxxxx).