Ban user from a specific group

Description

• Ban user from group.
• Group admin toggles the ban on/off, including "personal" groups, i.e. the victorwu namespace, and thus, controlling all projects there.
• User can still log into GitLab and has same read permissions as usual.
• When the user is banned from a given group, the user cannot do write operations on all projects within that group and that group itself.
• This includes creating issues, merge requests, participating in comments, pushing code, and so forth.

Out of scope

• No special notifications or user-facing UI for this initial iteration.
• Configuring a ban to expire later is a future feature.