Introduce list mode and permissions

[noyainrain]

Introduce a list mode which indicates how users work with a list and for that defines permissions, i.e. what users can do and the privileges of list owners.

A concept is available as part of the list feature draft.

[noyainrain]

Draft:

For now only collaborate and view mode and the currently implemented actions are relevant, the rest is for future reference.

class List:
    """
    .. describe:: mode

       List mode, describing how users work with the list. In ``collaborate`` mode, users
       collaborate on the list. In ``contribute`` mode, users contribute (items) to the list. In
       ``view`` mode, users only view the list.

       The following permissions apply:

       * ``collaborate``: Authenticated users may modify [1]_ the list and create, manage [2]_ and
         modify [3]_ items
       * ``contribute``: Authenticated users may create items. Item owners may modify [3]_ their
         items.
       * ``view``: None

       List owners always have full permissions.

       .. [1]: Edit, trash and restore the list and move items
       .. [2]: Edit tags of, check, uncheck and assign items
       .. [3]: Edit, trash and restore items
   """

    # _PERMISSIONS = {
    #     'collaboration': {
    #         'item-owner': {                                    'item-manage', 'item-modify'},
    #         'user ':      {'list-modify', 'list-items-create', 'item-manage', 'item-modify'}
    #     },
    #     'contribution': {
    #         'item-owner': {                                                   'item-modify'},
    #         'user ':      {               'list-items-create',                             }
    #     },
    #     'view': {
    #         'item-owner': {},
    #         'user ':      {}
    #     }
    # }

class Item:
    def check(): # ...
        """
        Permission: See :ref:`List` *mode*.
        """

UI draft:

• See Check item #2 (comment)
• If a user may not perform an action, hide the related UI element

[noyainrain]

Depends on noyainrain/micro#60.