Do not strip leading and trailing whitespace from raw posts

This is in response to the thread [Stripping Leading Whitespace Breaks Code Blocks at Beginning of Post](http://meta.discourse.org/t/stripping-leading-whitespace-breaks-code-blocks-at-beginning-of-post/3007) on meta.discourse.

Code blocks at the beginning of a post are broken because leading whitespace is stripped from posts' raw bodies. This patch removes the whitespace-stripping behaviour, but adds a new validator so that length limits are still enforced on posts as though the whitespace was  stripped.