Make Gitaly RepositoryExists opt-out

What does this MR do?

Changes the feature toggle status of the gitaly_repository_exists feature flag to opt-out. This means that by default, all GitLab installations will start using Gitaly to check if a repository exists.

Currently most GitLab installations do not make Gitaly calls from Ruby processes. This MR changes that; all GitLab 10.5 installations will start making Gitaly calls from Ruby by default. If there are any compatibility issues with Ruby and gRPC in some environments then this change will make those apparent.

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Why was this MR needed?

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
• End-to-end tests pass (package-qa manual pipeline job)

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