Footer: remove jsonp call

[stsewd]

Use normal json instead,
we don't have problems with CORS anymore and this allow us to cache the
footer.

[ericholscher]

Makes sense to me 👍

[davidfischer]

This will work as long as the proxied_api_host is the same as the host. Otherwise it won't work. Also, you can remove crossDomain: true, I believe.

[ericholscher]

We should probably just change the function name on the jsonp call to be consistent tho, and that will solve this issue with fewer changes. 👍

[ericholscher]

This looks like a better approach, and protects use cases that aren't doing proxied API. I wonder if the analytics endpoint should use jsonp for consistency?

[stsewd]

I wonder if the analytics endpoint should use jsonp for consistency?

The way I see it, jsonp is just a hack around CORS, I think new endpoints should use CORS rules to fix that.
We already do something like that here https://github.com/readthedocs/readthedocs.org/blob/master/readthedocs/core/signals.py#L36-L36