Amendments to the `annotate` command

This addresses the following:

• Fix annotation input only posting the first word
• The command now returns a confirmation over Slack
• Post annotation to specific dashboards only (by UID)
• Attach the user-annotation tag
• Did some house cleaning around the Grafana API integration

Command is now as follows:

bin/chatops annotate annotation text here --dashboards-uids A,B,C --<env>

Where A,B,C are UIDs of Grafana dashboards, which are part of their URLs.