Merge conflict in both directions design

• GitLab currently has a merge conflict UI editor.
• The feature allows users to resolve conflicts by adding a commit back to the source branch.
• This has caused some confusion https://gitlab.com/gitlab-org/gitlab-ce/issues/21947, regarding where the conflict resolution commit wold land. (Many folks think it would land in the target branch.)
• This issue https://gitlab.com/gitlab-org/gitlab-ce/issues/44895, will alleviate that confusion.
• But we also want to allow the conflict commit to be applied to the target branch as a feature.
• As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/21947, we want to allow merge conflicts to be resolved in both directions, i.e. to be applied to the source branch or target branch.
• This issue is to provide a UX design that achieves that.