Turn FluView into an indicator

[melange396]

We "should" be able to turn the data we have for FluView into a collection of covidcast-style signals.

FluView is our only API endpoint besides covidcast that appears to currently receive an appreciable number of requests. The current situation with it being its own endpoint works well (and i am not suggesting it should go away in the near future), but would be nice to be able to include FluView information in the covidcast endpoint alongside our other signals. That way, this data can share various benefits enjoyed by our active covidcast signals like: being easily comparable to other signals, easy(er?) incorporation into our covidcast dashboards, inclusion in the signal discovery app, etc.

I have not explored this dataset recently and there may be hurdles to overcome in making this a reality... We should determine feasibility and make it happen, if so! It can also give us an opportunity to incorporate our first signal with intrinsic demographic breakdowns.

FluView endpoint documentation: https://cmu-delphi.github.io/delphi-epidata/api/fluview.html
FluView endpoint implementation: https://github.com/cmu-delphi/delphi-epidata/blob/dev/src/server/endpoints/fluview.py

[nmdefries]

Might be nice to have demographic support (#347) in DB before doing this, but doesn't necessarily block.

[nmdefries]

Also see #180