Allow users to disable bans in certain groups

[davidmz]

Added

• Users can now disable bans in certain groups. One can disable bans in any group, no need to be an admin or even a member of the group.

  If user disabled bans in some group then:

  • He can see posts, comments, likes and comment likes in this group from users he banned.
  • If he is the administrator of this group, he can see posts in this group from users who have banned him.

  There is a new file, doc/visibility-rules.md, with description of this algorithm.

  API changes:

  • New methods POST /groups/:groupName/disableBans and POST /groups/:groupName/enableBans;
  • 'youCan' field of serialized group can have value 'disable_bans' or 'undisable_bans';
  • There are two new events in user notifications: 'bans_in_group_disabled' and 'bans_in_group_enabled'.

[davidmz]

This PR is safe for deploy, because it does not change the current behavior in any way without explicit action by the user. To allow users to do that, the UI needs to be updated.

[davidmz]

Я не отключал возможность комментирования для забаненного админа группы так как это противоречит остальным нашим правилам (например, админ может комментировать пост даже если его автор отключил комментарии).