Prior to this change, we were parsing `Post#cooked` every time we
serialize a post to extract the usernames of mentioned users in the
post. However, the only reason we have to do this is to support
displaying a user's status beside each mention in a post on the client side when
the `enable_user_status` site setting is enabled. When
`enable_user_status` is disabled, we should avoid having to parse
`Post#cooked` since there is no point in doing so.