FEATURE: Add group settngs to allow users to leave a group freely.

https://meta.discourse.org/t/split-join-leave-freely-setting-on-groups/65565

db/fixtures/002_groups.rb

@@ -7,8 +7,8 @@ Group.where(name: 'everyone').update_all(visibility_level: Group.visibility_leve
 
 ColumnDropper.drop(
   table: 'groups',
-  after_migration: 'AddVisibleBackToGroups',
-  columns:  %w[visible],
+  after_migration: 'RemovePublicFromGroups',
+  columns:  %w[visible public],
   on_drop: ->() {
     STDERR.puts 'Removing superflous visible group column!'
   }