admin: Allow user to enforce lookup type for field

Allow the user to enforce any of the following lookup types for fields
in model filters:

    'regex', 'iregex', 'month', 'day', 'week_day', 'search', 'contains',
    'icontains', 'iexact', 'startswith', 'istartswith', 'endswith',
    'iendswith', 'isnull', 'exact', 'gt', 'gte', 'lt', 'lte', 'range',
    'in', 'year'

For example, if we have two Astakos users:

    * first_name: John, last_name: Doe
    * first_name: john, last_name: doe

we can distinguish between them by doing a query like this:

    first_name__exact=John last_name__contains=Do