Use CorrelationID middleware in Pages

Since Pages does not appear to use CorrelationIDs are present (it should!)

The following discussion from !397 (merged) should be addressed:

• @andrewn started a discussion: (+2 comments)

  @jaime would you mind reviewing please?

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1. Add the correlation ID to Pages, including a propagate-correlation-id flag.
2. Add flag to Omnibus `gitlab_pages['propagate_correlation_id'] to Omnibus
3. Update Pages admin documentation

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Propagating the correlation ID

Setting the propagate_correlation_id in Omnibus will allow installations behind a reverse proxy generate and set a correlation ID to requests sent to GitLab Pages. When a reverse proxy sets the header value x-request-id, the value will be propagated in the request chain. Users can find the correlation ID in the logs.