FEATURE: constrain post_excerpt_maxlength (PR #13064)

Too long excerpts don’t make sense. They would make UI wonky. We already have a constrain for the topic_excerpt_maxlength setting. This adds the same constraint for post_excerpt_maxlength.

I also changed the max value from 999 to 1000. It won’t break anything. An existent bug would reproduce with both values anyway.

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