Host gitaly-proto inside gitlab-org/gitaly?

In the beginning, gitaly-proto was found at gitaly/proto. This got annoying so we broke out the proto definitions and their generated stubs into gitlab-org/gitaly-proto.

Sadly, the knowledge of exactly why this was annoying has been lost. I was there but I don't remember it clearly. So I suggest the following:

• we investigate what the consequences would be
• if it looks like an improvement we change the hosting of the proto
• if we change the hosting we do it in such a way that it's a "two-way door", i.e. so that we can go back to the current situation if we want