Ban user state and UI [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/327353

Introduces banned user state and UI. Adds the option to ban a user to the Admin Area > Users > (user's name) page. Clicking "Ban user" currently only blocks the user. In future iterations, banning a user will also hide their comments and issues from other users, as described in the epic proposal.

ban is behind a feature flag, ban_user_feature_flag, which will be removed when work on the ban feature is complete. Rollout issue: #330667 (closed)

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Testing with feature flag

# to enable banning:
pry(main)> Feature.enable(:ban_user_feature_flag)
=> true

# to disable banning: 
[84] pry(main)> Feature.disable(:ban_user_feature_flag)
=> true

Screenshots (strongly suggested)

With ban_user_feature_flag enabled:

Banning a user:

Banned user tab:

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With ban_user_feature_flag disabled:

The "Banned" tab no longer appears, but you can still filter by banned to find users that were banned before the feature flag was disabled.

Banned users can be unbanned, but cannot be re-banned until the feature flag is enabled again.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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