Add "experiments uptake" API

[mreid-moz]

We need an API to make the experiments uptake data available for use in experimenter et al.

A description of the desired API is in this doc.

[wlach]

Proposed approach:

• Add an ExperimentUptakeDatum model to https://github.com/mozilla/missioncontrol/blob/master/missioncontrol/base/models.py (base it off of the existing datum class)
• Add a new etl function to ingest this data from athena and add a management command to run it for debugging purposes. Prior art here: https://github.com/mozilla/missioncontrol/blob/master/missioncontrol/etl/experiment.py and https://github.com/mozilla/missioncontrol/blob/master/missioncontrol/etl/measure.py and https://github.com/mozilla/missioncontrol/tree/master/missioncontrol/etl/management/commands
• Add a new view to query this table at https://github.com/mozilla/missioncontrol/blob/master/missioncontrol/api/views.py
• Write some basic unit tests (as much as is sane to write without blowing your brains out) to test the above based on the prior art here https://github.com/mozilla/missioncontrol/tree/master/tests

[wlach]

My understanding is that we made telemetry-streaming post this data directly to test tube, so we don't need to do this in mission control anymore. Hurrah!