refactor: remove admin monkey patching

[dopry]

Description

• remove admin monkey patching.
• rename admin site classes to use the module namespace.

BREAKING CHANGE: Sites will need to explicitly use TwoFactorAdminSite or
extend the TwoFactorAdminSiteMixin

Motivation and Context

Monkey Patching the AdminSite isn't generally a good practice. We cannot know if users have custom admin sites that we may be breaking or are using other authentication modules that may impact admin login.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.