Handles inline editing of posts.

• Requires a valid session ID and topic ID in the URL
• If no message ID is in the URL, assumes you're modifying the first one from the specified topic
• Requires the modify_own permission to modify your own post
• Requires the modify_any permission to modify someone else's post if you didn't start the topic
• Requires the modify_replies permission or the modify_any permission to modify someone else's post if you did start the topic
• Validates the changes and handles errors as needed
• Sends the changes off to ModifyPost when you're done
• Uses either the "modifydone" or "modifytopicdone" sub-template when you're done modifying the post or the topic (respectively) if "xml" is in the URL