Resolve "New re-usable component for Blob File Content"

⚠ This MR is still behind the Feature Flag. To click-test, you should enable "Vueification" of the Snippets in Rails console (rails c):

Feature.enable(:snippets_vue)

What does this MR do?

1. Adds stateless Blob Content component that resolves New re-usable component for Blob File Content Issue
2. Finishes communication between different Blob-* components as the final missing bit for Refactor snippets view to Vue
3. Re-enables still-in-HAML components for Discussions and Reactions
4. As a side-effect, this MR also resolves #28819 (closed)

Known caveats

Both issues listed below are related to backend and will be handled in #205505 (closed). This MR is designed so that once the issues listed below are addressed from the backend side, FE will keep working without issues and without a need of any update or refactoring.

1. The syntax highlighting in snippets is not shown at the moment in most file types with an exception for .md. This is due to an issue in the existing GraphQL API endpoint.

2. The rich viewer (like preview for markdown or .svg snippets) doesn't work properly at the moment. Since this feature is behind the feature flag, we should tolerate this just fine for now.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

References #196797 (closed)