merge add and edit form builders for connections

It's a pain having two form builders that are almost identical.

To this to happen we need #1143158: use menu loader in hook_menu -- so we can use a page callback for the add menu item, create a mostly empty connection object, and give it to the form builder to start with -- same pattern as nodes with node_add().

And then after this, we can move on to #1100500: change connection name to machine name.