Unify GPC header support

[haaavk]

Right now Shynet sometime treat GPC header same as DNT header and sometimes ignores it.
GPC support should be somehow unified.
I see four approaches:

1. Always treat GPC as DNT.
2. Always ignore GPC.
3. Add settings per service to tread GPC as DNT.
4. Add global flag in django settings to treat GPC as DNT

I personally believe that option 3 is the best because it offers the most possibilities.
GPC is about selling and sharing user personal data. I suppose it doesn't forbid simple page view count.