Move "Confirm user" button into header of Admin area

📖 What does this MR do?

In preparation for #332846 (closed) this MR moves the Confirm user button from a card to the header. It also changes the Impersonate button to the default button style and moves it to the right of the Edit button.

💻 Local testing

1. Login as an Admin (root user in GDK)
2. Navigate to Menu -> Admin -> Users and note the username of one of the users
3. In your terminal in the gdk/gitlab directory run bin/rails console
4. Run the following commands
   • user = User.find_by_username('<user name>')
   • user.confirmed_at = nil
   • user.save
5. Navigate to Menu -> Admin -> Users and choose that user
6. There should be a Confirm user button in the header

📷 Screenshots

View	Before	After
Unconfirmed user
Unconfirmed user modal
Confirmed user

🚦 Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] 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