A commonly reported issue from users on Gitter is that they get an
AttributeError for an attribute that should be present. This is often
caused due to the fact that they used the `list()` method to retrieve
the object and objects retrieved this way often only have a subset of
the full data.

Add more details in the AttributeError message that explains the
situation to users. This will hopefully allow them to resolve the
issue.

Update the FAQ in the docs to add a section discussing the issue.

Closes #1138

Convert our manager usage to be done via type annotations.

Now to define a manager to be used in a RESTObject subclass can simply
do:
      class ExampleClass(CRUDMixin, RESTObject):
          my_manager: MyManager

Any type-annotation that annotates it to be of type *Manager (with the
exception of RESTManager) will cause the manager to be created on the
object.

Sometimes we have errors where we don't get a dictionary passed to
RESTObject.__init__() method. This breaks things but in confusing
ways.

Check in the __init__() method and raise an exception if it occurs.

Move the 'gitlab/tests/' directory to 'tests/unit/' so we have all the
tests located under the 'tests/' directory.