New Post page: Put Preview button back on the left.

There was a recent regression due to "float: right".
Center text input form elements in all 3 cases: New Post, SPV comment, excerpt comment.
Remove superfluous centered-contents CSS class.
Revert mistaken commit of _host.
Merge pull request #33 from Libertree/fix-post-and-comment-form-buttons

Fix post and comment form buttons