Add loading spinner to submit button

What does this MR do and why?

Adds a loading spinner to the submit button in the new vulnerability form.

Screenshots or screen recordings

How to set up and validate locally

1. You'll need an EE license.
2. Navigate to :namespace/:project/-/security/vulnerabilities/new using the Address bar
3. Fill the form and hit submit button. It should be disabled and a loading spinner should appear.

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 #352128 (closed)