WIP: [ticket/10715] "return to previous page" returns to approved post page

When approving posts from viewtopic, "return to previous page" returns to
the approved post page.
A new variable is sent (backtopost=1) throughout the URL so that after
going to MCP. In the MCP, after approving the post, we check backpost
and if we find it we could return to the approved post page.

PHPBB3-10715