Add confirm modal to reactivate user

What does this MR do?

Add the modal confirmation to unblocking the user. This will make it match all other areas on this page.

Note: Similar to work done for unblocking a user in !47442 (merged)

Test Locally

• User index area: Visit admin/users area and click on any inactive user to deactivate/then reactivate - similar to http://127.0.0.1:3000/admin/users/asdfasdf
• User show area: Visit admin/users area and click deactivate on any inactive user, from this URL http://127.0.0.1:3000/admin/users, then visit the deactivated tab and reactivate them.

Screenshots (strongly suggested)

User index area:

before	after
Screen_Recording_2020-11-20_at_9.11.06_AM	Screen_Recording_2020-11-20_at_9.04.29_AM

User show area:

Before	After
Screen_Recording_2020-11-20_at_9.10.18_AM	Screen_Recording_2020-11-20_at_9.05.36_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