Send Slack message on user signup

[zacwood9]

To ensure that Go is not being abused, Go sends the SRCT Exec Board Slack messages when some actions are taken. Right now, this is done when a user adds a link, but it should also occur when a user signs up.

There are a couple utility functions in utils.py that enable this. Add another function to build a message for user signup, and send use the send_slack_message to send it when a user signs up.