Add status to needs

This change adds an extra `status` attribute to needs.

Status can be:
```
{ "description": "proposed" }
```
or
```
{ "description": "out of scope", "reason": "out of scope reason" }
```
It's designed to completely replace the `in_scope` and `out_of_scope_reason` attributes, so Maslow will need an update.
Is there any way to get rid of this embedded ID? It looks like there’s some [back and forth](https://github.com/mongoid/mongoid/pull/644) over this, but I don’t know what the latest state of it is. Either way, is it worth looking at whether you can take this out without breaking things?