[bugfix] support endless polls, and misskey's' method of inferring expiry in closed polls

[NyaaaWhatsUpDoc]

Description

As the title says really. Misskey doesn't always set the endTime AS property, both in the case of an endless poll (where closed is also unset), or in the case that an expiring poll is now closed and it uses the set closed property to infer the past expiry. As such there's a required database migration in order to support empty endTime property.

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• 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.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.