Add migration and spec

commit 3cc12e1268a6865f524d8fab1804f018312fdf5a
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Tue Jan 8 19:34:31 2019 +0900

    Add changelog to this change

commit 5006fc96e38db514956a35f53ae8ee536548a2e9
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Tue Jan 8 19:32:56 2019 +0900

    Remove partial index from ci_builds artifact_file

    ok

    Update schema

commit 3c956bdc02b195bc67d0327bf3748a631ea5466d
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Mon Jan 7 21:41:27 2019 +0900

    Add frozen_string_literal: true

commit 8c827cd616890160e6e8908843403a6f20c03236
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Mon Jan 7 21:40:06 2019 +0900

    Set batch size 100

commit aeee559777d3bdeadfd2b9bb61d460f2dc1fa8a6
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Fri Jan 4 18:33:05 2019 +0900

    Cleanup legacy artifact background migration