Previous code would not work with `disabled?` because that method would
send two parameters (second always `nil`) which we are not mocking.
Instead of mock yet another state, I decide to fix it where it belongs.
The GitHub importer (and probably other parts of our code) ends up
calling Feature.persisted? many times (via Gitaly). By storing this data
in RequestStore we can save ourselves _a lot_ of database queries.
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/39361
