compute: use authenticated endpoint via POST method

Created by: rvantonder

Apparently doing:

	base.Path("/compute/stream").Methods("GET").Name(ComputeStream)
	base.Path("/compute/stream").Methods("POST").Name(ComputeStream)

overwrites the GET handler. I should have done

base.Path("/compute/stream").Methods("GET", "POST").Name(ComputeStream)

Fixed in https://github.com/sourcegraph/sourcegraph/pull/35765/commits/22db5ff5eca027dfe85c7354ce9add7c248bf16c

Test plan

Tested manually for experimental feature. This time doesn't break GET handler.