Allow PostReceivePack to be enabled with Gitaly

Conversation: gitaly#353 (closed)

What does this MR do?

Reestablish the ability to enable PostReceivePack being processed by Gitaly

Are there points in the code the reviewer needs to double check?

No. This is not enabling the feature, it's making it opt-in.

Why was this MR needed?

The problems that prevent this feature from being enabled were fixed.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes gitaly#246 (closed)

/cc @andrewn @jacobvosmaer-gitlab