Add background migrations for filling nullified `file_location` on `ci_job_ artifacts` table

Description

In GitLab %11.3, we created the column file_location in ci_job_artifacts table. Here is the current status,

• Job artifacts created before %10.3 -> file_location is 1
• Job artifacts created after %10.3 -> file_location is NULL

We should fill file_location into all rows, and set NOT NULL constraints to the file_location column.

Also, we can remove this code which created for the transition period.

    def hashed_path?
      super || self.file_location.nil?
    end