Conversation OperationService::UserRebase

Should be migrated along with EE::MergeRequest#rebase_in_progress? (#865 (closed))

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Stages:

• RPC Design: #862 (closed)

• ~"Client Implementation": #863 (closed)

• Server Implementation: #864 (closed)

• ~"Acceptance Testing": #925 (closed)

RPC Endpoints:

• Endpoint::Name

Known Client Routes:

• Known client endpoints

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