Backport "Add a spinner for a loading elipsis menu" to 18.0

What does this MR do and why?

Describe in detail what merge request is being backported and why

As part of the pilot backporting every bug fix to n+2 versions, I'm backporting a fix for a blob overflow menu: Add a spinner for a loading elipsis menu. This fixes shifting UI on loading a blob page. I'm only backporting to %18.0 since the fix landed in %18.1 and the feature flag for this UI was enabled globally in %18.0

Resolved #537448 (closed)

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

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