Users can now use `scripts/regenerate-schema` to create a clean
`db/structure.sql` for their branch.

What the script does:
1. Disable the migrations (move them where Rails would ignore them)
1. Check out out a clean `db/structure.sql`
1. Reset the test database to this schema
1. Enable the migrations again
1. Run the migrations on the test database

This recreates `db/structure.sql` with the migrations added in the
current branch.

There are several methods the script uses to check out the schema file
from the target branch:

- When `CI_MERGE_REQUEST_TARGET_BRANCH_SHA` is set, it assumes it's
  running from CI and it will `curl` the file from `CI_PROJECT_URL`
  because it assumes a shallow clone is in place.
- You can set `TARGET`, like `TARGET=stable scripts/regenerate-schema`
  to set the target branch. In this case `git checkout $TARGET` is
  used to checkout `db/structure.sql`.
- Otherwise `master` is assumed and used to `git checkout`.