Improve consistency of admin user dropdown actions

📖 What does this MR do?

This MR is preparation for #332846 (closed). In that issue we will be moving the admin actions on the individual user view from cards to the same dropdown used by the user list view. To prepare for that this MR does the following:

1. Adds missing information about what each action does
2. Adds confirmation modals to the Approve and Reject actions
3. Fixes the button variants to conform to Pajamas design system action hierarchy guidelines
4. Removes an unnecessary div around the dropdown items to improve the semantics of the markup

💻 Local testing

1. Login as an admin (root on GDK)
2. Navigate to Menu -> Admin -> Users
3. Use the dropdown on the right of the table to select an action

Banned action

The banned action is behind a feature flag. Currently banning a user does the same thing as blocking a user, but in the future it will remove all comments, issues, MRs, etc.

• bin/rails console
• Feature.enable(:ban_user_feature_flag)

📷 Screenshots

Action	Individual user view	Users list before	Users list after
Activate
Approve		N/A
Ban
Block
Deactivate
Reject		N/A
Unban
Unblock

🚦 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