Replace JS modals with panel messages

[lannyheidbreder-mc]

I've worked with @brendonor (he wrote the code, I reviewed and gave feedback) to replace the JS modal alerts with in-panel messages. We have a branch that looks good to me, but I'm unable to submit a pull request because I'm not a collaborator on this repo. Let me know if you'd like to make it so we can create a PR, or anything else you'd like us to do.