Bug status label should be mutually exclusive

[karlcow]

Currently we have

1. needdiagnosis
2. (needcontact) See Make "Need Contact" label and reflect in the UI on webcompat.com #434
3. contactready
4. sitewait
5. fixed, duplicate, wontfix, worksforme, ?

It should not be possible to have these mutually selected. The proposal would be that when you select one, you unselect the other(s).

[karlcow]

A possible issue that might come, is that this constraints do not exist into GitHub. So webcompat will be more restricted. We might want to have namespaces for labels too. See #436

[hallvors]

Nobody minds if I take this..?

[miketaylr]

Sounds good!

(I would say generally if nobody is assigned, feel free to assign yourself--unless somebody didn't get assigned and it looks like they're committing code, etc).