30% Allow tablespecs to have filters

30% PR to test out the approach.

Tablespecs don't have filters. And on the user profile report, there are users that have signed in, but haven't created a profile - so on a tenancy with lots of users, there are loads of seemingly empty rows.

The tablespec for filters could look similar to the way they happen in search - but seeing as you're already working on a collection, you don't need to know where it's from - something like this:

```
"filter":[
  {
    "spec:email":{
      "$exists":true
    }
  }
]
```