Engaging Crowds: add completion counts to the subject set picker

[eatyourgreens]

Package
app-project

Is your feature request related to a problem? Please describe.
When I'm selecting a subject set to work on, I should be able to see which sets are already complete (or nearing completion.)

Describe the solution you'd like
We can get the number of available subjects, per set, from the Cellect API so let's use that to show subject set completeness.
Eg. https://cellect.zooniverse.org/workflows/16807/status
returns this response:

{"name":"16807","state":"ready","grouped":true,"prioritized":true,"pairwise":false,"subjects":112,"users":0,"groups":{"82738":101,"85304":11}}

response.body.groups is keyed by subject set ID and contains the number of available subjects for each set.

[eatyourgreens]

The numbers here are the number of subjects that haven't been retired for each subject set. If you're logged in, the count won't tell you whether or not you've personally completed a particular set.

[eatyourgreens]

Longer term, the subject set counts should be available from the Designator API, once prioritised selection is supported by Designator. https://designator.zooniverse.org/api/