Add group packages page

What does this MR do?

• Move Dependency Proxy feature from Group -> Overview tab to Group -> Packages tab
• Add Group -> Packages -> List page with all packages from all projects under the group. Same as Project -> Packages page but with project names and without remove controls.
• Add development fixtures for NPM packages

NOTE: Please merge https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29592 after this MR

Screenshots

Group -> Packages -> List

Group -> Packages -> Dependency Proxy

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