ENH: Improve topic split user experience

Clicking the split button on a topic should bring up a modal dialogue with instructions and side-by-side panels.

Start with snippets of each reply on the left and you move the ones you want from left panel to right panel.

When you have it how you want it, select target forum and click "save" to "complete the split".

Also want an option to add a (editable) postscript to the original post with a link from the original to the new topic. And a preamble (editable) to the new topic to point back to the original post. That way you can split but retain the context.

Probably not high priority and not getting done until we relaunch with an mvc version😅