Fix Spring Reloading AdminMode::Client

What does this MR do?

As described here: gitlab-development-kit#852 (comment 339013137)

I've been getting errors while running rspec tests with Spring such as the following:

Backtrace

     ArgumentError:
       A copy of Gitlab::SidekiqMiddleware::AdminMode::Client has been removed from the module tree but is still active!
     # ./lib/gitlab/sidekiq_middleware/admin_mode/client.rb:11:in `call'
     # ./lib/gitlab/sidekiq_status/client_middleware.rb:10:in `call'
     # ./lib/gitlab/sidekiq_middleware/duplicate_jobs/client.rb:9:in `call'
     # ./lib/gitlab/sidekiq_middleware/worker_context.rb:9:in `wrap_in_optional_context'
     # ./lib/gitlab/sidekiq_middleware/worker_context/client.rb:18:in `call'
     # ./config/initializers/forbid_sidekiq_in_transactions.rb:36:in `block (2 levels) in '
     # ./app/models/concerns/update_highest_role.rb:29:in `block in update_highest_role'
     # ./lib/after_commit_queue.rb:25:in `instance_eval'
     # ./lib/after_commit_queue.rb:25:in `block in run_after_commit_or_now'
     # ./config/initializers/forbid_sidekiq_in_transactions.rb:8:in `skipping_transaction_check'
     # ./lib/after_commit_queue.rb:24:in `run_after_commit_or_now'
     # ./app/models/concerns/update_highest_role.rb:24:in `update_highest_role'
     # ./config/initializers/forbid_sidekiq_in_transactions.rb:50:in `block in committed!'
     # ./config/initializers/forbid_sidekiq_in_transactions.rb:8:in `skipping_transaction_check'
     # ./config/initializers/forbid_sidekiq_in_transactions.rb:50:in `committed!'
     # ./ee/spec/controllers/projects/feature_flags_controller_spec.rb:9:in `block (2 levels) in '
     # -e:1:in `'

Following the suggestion from @engwan (gitlab-development-kit#852 (comment 339169310)), I've prefixed all the classes and modules here with ::. This seems to fix the issue.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
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• Style guides
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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