Introducing the icebox

[RookieRick]

I've added a new label "icebox" that we will use to help us (hopefully) keep GitHub Issues in sync with our internal Jira tickets.
With the "triaged" label my goal was to ensure we have 1:1 correspondence between Flask-Rebar Issues on Github and Jira tickets on our team's board within Autodesk. The downside to that approach has been that we have some long-standing Issues that are unlikely to percolate to the top of anyone's todo list without a clear business case compelling them. In Jira we use an "icebox" status for things like this so I'm just going to mirror that here.
If you see an Issue labeled "icebox" and it's something you want to work on, please feel free (and let me know)! Iceboxed issues are still "valid" (we have a separate label for "won't do")