Add NPM registry support to GitLab packages

What does this MR do?

Add NPM registry support to GitLab packages:

1. A user should be able to upload new packages to GitLab via npm publish.
2. A user should be able to install scoped package dependency from GitLab via npm install.
3. Authentication is done via oauth token. npm login is not supported at this stage.

Implementation

This MR adds 3 API endpoints:

1. GET packages/npm/:package_name allows NPM to receive information about the package and all available versions.
2. GET projects/:id/packages/npm/:package_name/-/*file_name allows NPM to download a package file.
3. PUT projects/:id/packages/npm/:package_name is where npm will send JSON about uploaded package including base64 encoded content of package itself

Screenshot

Existing UI was reused 100%.

What are the relevant issue numbers?

#5934 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #5934 (closed)