App config form improvements

[skkra0]

• Redirect to the app page when an app was created, even if the first deployment then fails before build
  • Ensure user can always retry if the first deployment fails before build
• Show an error if app group name exists
• Check that a selected workflow can actually be triggered on the selected branch

[FluxCapacitor2]

I'm going to leave the last item (checking if the workflow can actually be triggered on the selected branch) out of this PR since it involves a lot of moving parts. It would be somewhat difficult to write a perfectly-accurate matcher that looks at branches and branches-ignore on all the workflow trigger types.

https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#onpull_requestpull_request_targetbranchesbranches-ignore