Working to reduce CE-to-EE merge conflicts

This is a meta issue to track ideas for improvements we can make to the CE-to-EE merge process to make it less of a never-ending nightmare.

• Look into activerecord_sane_schema_dumper to eliminate whitespace-only changes to the db/schema.rb file: gitlab-org/gitlab-ce!6827 & gitlab-org/gitlab-ce!6827
• Add a CI task to check if a merge request in CE will cause a conflict when merged into EE. Depends on "warn on fail" CI functionality: gitlab-org/gitlab-ce!6746
• Use a webhook and the API to automatically create MRs against EE whenever an MR is merged into CE, with automerge enabled automatically, so that a dev only needs to take action in the result of a conflict or a failing build.
• Document which specific files routinely see merge conflicts. Useful information would be how often and how difficult they are to resolve. This should be done every time a conflict is encountered, no matter how small: https://docs.google.com/spreadsheets/d/1A4f9UwGuJgyOTZSs-7qU1beLU_ivfwgpvk6wmlhb9GE/edit#gid=0