Conversation RepositoryService::BackupCustomHooks

Spun off from #1195 (closed).

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Stages:

• RPC Design: #1224 (closed)

• ~"Client Implementation": #1227 (closed)

• Server Implementation: #1225 (closed)

• ~"Acceptance Testing": Create Issue

RPC Endpoints:

• RepositoryService::BackupCustomHooks

Known Client Routes:

• rake gitlab:backup:create

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