Add loading blob viewer

What does this MR do and why?

This MR adds a client-side loader (relevant ticket #324322 (closed)).

This MR is part of the effort of refactoring the repository browser into a Vue app.

Screenshots or screen recordings

How to set up and validate locally

This feature is behind the :refactor_blob_viewer feature flag.

1. Run rails c in the project root.
2. Run Feature.enable(:refactor_blob_viewer) in the rails console.
3. Upload the appropriate file type.
4. View the newly uploaded file and ensure that it works as expected.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #324322 (closed)