Fixed pos flash message, scroll to prev pos, fix comment delete on user prof

Made the flash message fixed position and after a form post the page will be scrolled back to the last window position the user was at.

When a form is submitted and the user is scrolled down the page (like for deleting a comment) it's jarring to be scrolled back to the top of the page. To remedy this the flash message is placed in fixed position and the user is kept at their previous scroll position after a POST.

Also fixed comment delete on the user profile page.