[chore/bugfix/horror] Allow expires_in and poll choices to be parsed from strings

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

Unfortunately, some of our fave clients (pinafore, semaphore, derivatives thereof) submit some poll-related fields as strings when they ought to be integers. These clients aren't maintained anymore, so submitting fixes upstream is not useful. This hack allows those strings to work as well. This is technically compliant with the Mastodon API, though the Mastodon API is not documented as allowing this.

Checklist

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If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• 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.