Add some strings that were missing in gitlab.pot

Bob Van Landuytrequested to merge
bvl-add-missing-externalized-strings into master

This just adds some externalized strings that weren't added to gitlab.pot yet.

Should make reviewing things like https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17395 easier.

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/39292

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