Fix markdown length validation for exit pages

[thomasiles]

When the char count for the markdown field is too long, the error cannot be displayed because the translations assumed the count field was available.

As the error is checked in the markdown gem and added to the model in the our validator, it doesn't set the count field.

I looked at different ways of making the count available. I don't think it's worth making wider code changes though. Instead, I've hardcoded the 5000 character limit in the translations.

What problem does this pull request solve?

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