Update delete user modal manager

What does this MR do?

This MR updates the delete modal manager to add event listeners on matching buttons like we do with confirm_modal, rather than the document.

This is a needed change to complete #285110 (closed) since GlDropdownItem's deep node branch isn't compatible with the current implementation.

Screen recording

No visual change. Works as usual.

Admin/Users
Kapture_2021-02-05_at_14.00.44

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - No visual change
• [-] Documentation (if required) - No visual change
• 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 #285110 (closed)