Apply feedback for pull request #278

https://github.com/python-discord/site/pull/278

This commit applies the feedback given to PR #278. I have made a
number of significant changes:

- I have added tests for the `MigrationsTestCase` class, including a
  test to see if the test case travels to the specified points in the
  migration history.

- The dictionary of infracted users in the active migration tests now
  uses informative strings as keys. This makes it easier to relate a
  specific test to the data it's testing.

- I have simplified the migration query logic by replacing unneeded Q
  objects by simpler keyword arguments to `filter` method calls.

- I have removed the custom validation logic for allowing only one
  active infraction of a given type per user and replaced it by a
  `UniqueTogetherValidator` with a custom validation error message.

- `test_unique_constraint_accepts_active_infraction_after_inactive_
   infraction` relied on an Error results instead of a Fail result
   to indicate an issue with the database constraint. Since a fail
   should lead to a Fail status, I've replaced it by a try-except
   block that signals a test failure if an `IntegrityError` was
   caught by it.

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