create TriageNurse plugin

[monkeypants]

after #9, the SortingHat plugin will fill the intray of the TraigeNurse.

So we want a TriageNurse plugin that periodically nags the Triage nurse (etc) when there is sorting to do. Requires more analysis, but for example:

• notify slack (traige_nurse channel) when new tickets are in the sorting hat
• notify slack (direct message to Triage Nurse) gentle reminder about action required
• notify slack (general channel) mild panic when very old tickets need sorting

This plugin requires some parameters in .gnome.yml:

• the intray milestone to monitor (e.g. "Sorting Hat")
• Botty McBotFace endpoint (errbot)

And the Botty McBotFace Bot service needs configuration to know stuff like:

• the list of triage nurse(s)?
• triage_nurse channel?
• errbot service endpoint?