Create BannedUser table

What does this MR do?

Introduce BannedUser table. Following some discussion and testing done by @krasio, it was determined that having a dedicated banned_users table would be the most optimal way to record banned users (introduced by !61292 (merged)). Update the app so that when a user is banned, a new record is inserted into banned_users (and the opposite - when user is unbanned, the record is removed). Banned user contributions will be hidden in a future iteration (!66687 (merged) (WIP))

Migration output:

% rails db:migrate 
== 20210623182010 CreateBannedUsers: migrating ================================
-- create_table(:banned_users)
   -> 0.0111s
== 20210623182010 CreateBannedUsers: migrated (0.0111s) =======================

% rails db:rollback
== 20210623182010 CreateBannedUsers: reverting ================================
-- drop_table(:banned_users)
   -> 0.0026s
== 20210623182010 CreateBannedUsers: reverted (0.0026s) =======================

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Does this MR meet the acceptance criteria?

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