Add Ban/Unban actions to user administration dropdown in the admin area

📖 What does this MR do?

Adds a Ban and Unban action to the user administration dropdown in Admin -> Users.

This is in preparation for #332846 (closed) where we will be moving the user administration actions that are currently in cards to a dropdown.

Currently the Ban and Unban actions are behind the ban_user_feature_flag feature flag which is defaulted to off. At this time banning a user only blocks the user but in the future it will remove all comments, issues, MRs, etc.

💻 Local testing

1. Enable the ban_user_feature_flag.
   • bin/rails console
   • Feature.enable(:ban_user_feature_flag)
2. Click Menu (in top nav) -> Admin -> Users
3. Click the dropdown with a cog icon in a user row

📷 Screenshots

View	Before	After
Unbanned user
Banned user
Ban user modal	N/A ______________________________________________________________
Unban user modal	N/A ____________________________________________________________

🚦 Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • Not needed, this change is behind a feature flag
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
  • Docs will be updated when #332846 (closed) is completed
• 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