[chore] Increase default max image description to 1500 chars, collapse cw char count into status

[tsmethurst]

Description

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This pull request removes some of the friction of using Mastodon clients on GtS by bringing some of our character counts more in line with Mastodon:

• Increase default image description char count from 500 => 1500 (the Mastodon default). There's no real reason we had such a limited default in the first place.
• Remove the statuses-cw-max-chars config option, and just add cw and body text together when checking that a status is within character limits. This is also closer to the Mastodon behavior, and so should cause fewer issues in clients (like Tusky, for example) which provide a total character count that includes the cw. Technically this is a reduction in how many characters are permitted by default in a GtS status, but admins can always bump their character count up to 5500 or something if they want to.

Partially addresses #2457, but doesn't quite close it since we don't count links as 23 characters etc.

Checklist

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• 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 not leveraged AI to create the proposed changes.
• 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.