Disable code hints when multiple selections are active

This is a short-term workaround for #7625.

Currently, if multiple selections are active, code hints appear at the primary selection. That seems okay, but if you then choose a hint, we only insert it at the primary selection. This feels broken, because it's a typing/insertion operation, so the user would expect it to insert at all selections. I've run into this many times myself.

I've added https://trello.com/c/F567vo1F to Nominations for making code hints actually work with multiple selections.