Allow whitespace in duration strings and update tests

https://github.com/python-discord/bot/issues/446

After review feedback and a discussion in the dev-core team, I've
changed a couple of things:

- Allow a space between amount and unit in the duration string;

- Allow a space between different units in the duration string;

- Remove the old ExpirationDate converter completely;

- Remove the dependency `dateparser` from the Pipfile;

- Update tests for the two types of optional spaces;

- Change the test for valid cases to a more readable format;

This PR closes #446

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