Restrict which characters a user can view
Refactor code for returning a list of characters

Push code for determining which characters a user can view into its own private
function.
Limit who can view each individual character.

Ensure that only specific users can view each character by adding an
authorization check to the character/<id>/ path. Add a unit test to ensure this
logic is working.
Update tests.

Write a doctest for function `views._viewable_characters`. Also update the test
case `test_views.CharacterTestCase`.