new access handeling

Ditches hook_permission in favor of custom access handling - mobwrite_node_update_access() and an accompanying function, mobwrite_share_mode(). Seems to be in order, but some issues remain:

mobwrite_share_mode() should removed existing grants when changing modes. For example, if I user is sharing editing in URL Mode, the function handles granting users access when they have the proper URL key. But if the user switches to "no access" or stops the sharing, then the mode is changed, but the previous participants still have access. I think I need to just remove all the participants from the table...

Non-mobwrite users that have access to update the node, still have access even if a mobwrite session is on-going. Their update will be over writen but the mobwrite session, but we could lock them out #870638: No auth user lock out.