Compress images

What does this MR do?

EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23902

Compress all images to be included in 11.6.

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards