Need to add docs_failed_cr_target to XDCR Mutations Skipped stats info

For MB-50088, XDCR has added the stat "docs_failed_cr_target" to complement the existing "docs_failed_cr_source" info.  The UI needs to add this info to the XDCR Mutations Skipped stats.

docs_failed_cr_source is currently shown in the UI stats for XDCR as "XDCR Mutations Skipped", but as MB-50088 points out, "docs_failed_cr_target" should also be included in that info (since it's immaterial to the user whether the mutation wasn't applied on the target due to conflict resolution check done on the source or the target cluster).

XDCR Mutations Skipped is shown in 1) XDCR Stats under the XDCR tab in the list of Outgoing Replication and in 2) the Dashboard --> All Services --> XDCR.