api/quota: add endpoint for returning user quota

It's nice to change it, but how do I know the current value? :)

Related to tsuru/tsuru#869.
api/quota: add endpoint for returning app quota

Same as 557d372117b363b438ac935d80f948bd00a89dfb.

Related to #869.
docs/releases/tsr: admins can also view quota stats on 0.7.0 :)

Related to #869.