Default to Gitaly for 'git push' HTTP/SSH, and make Gitaly mandatory for SSH pull

What does this MR do?

• Make Gitaly mandatory for 'git pull' over SSH
• Make Gitaly opt-out for 'git push' over HTTP(S) and SSH

Pull-over-SSH has been in opt-out since GitLab 10.3 and we have heard no complaints.

Push-over-HTTP/SSH has been running on gitlab.com without trouble since mid December 2017.

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

Why was this MR needed?

Our goal is to remove all Gitaly migration flags, this is a small step in that direction.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

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