Migrate user admin block user modal to GlModal

What does this MR do?

Migrates to use the confirm modal for ease of updating in the future.

Note: no changelog since this was mostly a developer facing change.

Test Locally

• Individual User: Visit admin/users area and click on any user - similar to http://127.0.0.1:3000/admin/users/asdfasdf
• User Index: Visit admin/users area and use dropdown on any user - http://127.0.0.1:3000/admin/users

Screenshots (strongly suggested)

Same as before - background development work only.

Individual User

Screen_Recording_2020-11-11_at_1.05.54_PM

User index

Screen_Recording_2020-11-12_at_10.18.08_AM

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 #267118