When the workspace owner is changed, the new owner is not added as a member nor in the admin group

This issue is about changing the owner of the workspace from the Workspaces administration (Administer Workspace -> Configuration -> Workspace), not the owner change in the old wiki manager UI. While both changes should be taken into account, since we consider wiki manager deprecated, we could accept misfunctions about the old UI.
Note that in order to experiment this issue, you need to use some sort of patch for XWIKI-8396 since for now owner change is not possible.

When a user is selected as the workspace owner upon workspace creation, he is automatically added in the members of the workspace and in the admin group of the workspace. When the owner is changed to a different owner nothing changes in the members group (XWikiAllGroup) nor in the admins group (XWikiAdminGroup), however the owner has rights in the workspace since the owner has special rights anyway. Note that old owner keeps administrative rights on the workspace since he remains a member of the local XWikiAdminGroup.

Two things should happen:

1. the new owner should be added to members if he's not already there and to admin group as well
2. the old owner should be removed from the admin group automatically (but not from the members). This is tricky since one might want to preserve the administrative rights of the old owner (thus not remove him from admin group) but I prefer automatic removal since adding is automatic upon workspace creation: the creator of the workspace is not aware of the fact that the owner is added to the admin group, so he will not be aware of the fact that changing the ownership will not remove the user's administrative rights.