Add post-deployment migration system (#8182)

Adopted from GitLab CE. Generate new migration with:

    rails g post_deployment_migration name_of_migration_here

By default they are run together with db:migrate. To not run them,
the env variable SKIP_POST_DEPLOYMENT_MIGRATIONS must be set

Code by Yorick Peterse <yorickpeterse@gmail.com>, see also:

https://gitlab.com/gitlab-org/gitlab-ce/commit/83c8241160ed48ab066e2c5bd58d0914a745197c

1 files changed, 0 insertions, 0 deletions

diff --git a/db/post_migrate/.gitkeep b/db/post_migrate/.gitkeep
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/db/post_migrate/.gitkeep