Update data/release_posts/unreleased/2019_09_22_gitlab_12_3_released.yml

Tasks

• docs links (MR - WIP, still needs feedback to be improved)
• tiers
• screenshots for post and features.yml (< 150kb and according to guidelines)
• docs in review - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32942
• @valexieva - to update all references from 12.2 to 12.3
• added to features.yml

cc: @ebrinkman for visibility since this is my first release post