Refactor: automatically get values for MANAGER_ATTRS, QS_ATTRS, etc.

[atodorov]

From the bottom of #299 (comment)

MANAGER_ATTRS, QS_ATTRS, MODELADMIN_ATTRS, MODEL_ATTRS, FIELD_ATTRS and the rest of the lists containing auto-generated attributes from Django can probably be generated at runtime instead of hard-coding them one by one.

transforms/foreignkey.py and checkers/foreign_key_strings.py already call django.setup() and README already asks users to specify DJANGO_SETTINGS_MODULE=your.app.settings before running pylint-django.

[atodorov]

Depends on #290