Resolve "Add a loading state to the MR File Tree"

What does this MR do?

Adds a loading spinner while the tree data hasn't been set yet.

This prevents the "No files found" to display briefly while the data is being set.

Note: this MR is an iterative step to improve this very small detail before we ship the startupjs. It's still noticeable there.

Screenshots

Kapture_2020-07-09_at_14.26.30

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

Closes #227588 (closed)