Prevent creating multiple ApplicationSetting by forcing it to always have id=1

Open Kamil Trzcińśki requested to merge prevent-creating-multiple-application-settings into master

What does this MR do?

This approach prevents writing duplicates. We force that every new object created to have id set to 1 which is primary key for the table.

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/34728

cc @stanhu @nick.thomas @yorickpeterse