Use merge_db_schema for db/schema.rb conflicts

What does this MR do?

This adds the use of merge_db_schema for resolving merge conflicts in db/schema.rb. To use it you still need to initialise the configuration, which can be done by running:

merge_db_schema-init --force

With this MR in place, we can have the CE upstream code make use of this Gem to reduce the number of conflicts in db/schema.rb, hopefully reducing the time these upstream MRs take to merge.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/release/framework/issues/42

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides