Longer passwords and passphrases are much more common than when post passwords were introduced all those eons ago, so let's increase the length of the `post_password` field from 20 to 255 characters.
The password will continue to be stored in plaintext, as the ability for the post author to view the password is a commonly used feature.
Trivia: this was the last 3 digit Trac ticket to be closed.
Props skippy, nazgul, iandunn, adamsilverstein, pento.
Fixes#881.
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