Register "gitaly_delta_islands" feature flag

What does this MR do?

Gitaly server side feature flags do not work unless they are white-listed in Gitlab::GitalyClient. We created a feature flags for delta islands gitaly#1575 (closed) a while ago already, but we forgot to register the feature flag in gitlab-ce. This MR fixes that.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

This change has no immediate production impact because the feature flag it registers is off by default.