Add a loding indicator for the submit button in the CI lint page

What does this MR do?

⚠ This change is behind and FF, more at: #249661 (closed)

Add a loading indicator for the submit button.

In large CI configurations, the time to give feedback can increase, so users might think the UI is not responding as the validation occurs.

When the user ask to lint a CI config, the feedback may take over a few seconds, they should get feedback that the system is working.

Screenshots (strongly suggested)

Note the "Validate" button, now it has a loading icon when the user ask to load.

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

Related to #249661 (closed)