Add depedency proxy models

What does this MR do?

Add AR models for dependency proxy feature. Because original MR is quite big I decided to extract few things into separate merge requests.

Issue - https://gitlab.com/gitlab-org/gitlab-ee/issues/7934

Implementation details:

1. Group settings to enable/disable feature per group. In future it will be has_many relationship with extra settings like registry URL
2. Dependency proxy will cache blobs with DependencyProxy::Blob model. Feature is done on a group level so blob belongs to group.
3. DependencyProxy::Registry right now locked into docker hub. Once we support different registries, this model will evolve.
4. Database migrations for those models are merged in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10676
5. For more context about how this code will be used see https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9750/diffs.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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
• Security reports checked/validated by a reviewer from the AppSec team