Allow users with edit rights on a group's document to edit the group members

Currently, XWiki groups can be edited only by wiki-level admins.

The current improvement proposes to allow non-admin users to act as group managers (or moderators) by being able to add or remove users to a group.

Adding such a group manager implies simply giving edit rights to a normal user.

Technically, the XWikiGroupSheet needs to be altered to allow not only admin users to see the "add user to group" control and the livetable macro needs to be improved so that actions are displayed not only to wiki admins, but also to users that have edit rights on a row(document).