Allow users to rebind the shortcuts from their user profile UI

Shortcuts are currently defined for all users of XWiki the same way.
However it might happen that some shortcuts are overridden by users' own shortcut.
As it's almost impossible to know in advance which shortcut could be use by the different users, we should just allow them to rebind them the way they want in their own profile.
The users are already allowed to rewrite their shortcuts by creating a dedicated page, so we should have a dedicated UI for that