[bugfix] Return all accounts when list accounts limit <= 0

[tsmethurst]

Description

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This pull request updates our /api/v1/lists/{id}/accounts endpoint, to return all accounts in the given list when limit is explicitly set to 0 (or less). As odd as this behavior may seem, it is in line with Mastodon API expectations, and without it clients like Tusky (which set the 0 limit explicitly) show only one account in a list, regardless of how many are actually present.

closes #2006

Checklist

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• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
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