The "scm buttons" (Git, Github) at the top of creation should disabled once creation begins.

Per discussion here: https://github.com/jenkinsci/blueocean-plugin/pull/869#issuecomment-283495039

Repro Steps:

1. Click "New Pipeline"
2. Select "Github"
3. Enter Github access token (if needed)
4. Select an organization
5. Select "auto discover"
6. Click "Create Pipeline"

Expected Results

1. All form elements are disabled until creation completes (either successfully or unsuccessfully).

Actual Results

1. SCM buttons "Git" and "Github" remain enabled. If clicked, they will wipe out the current creation flow.

Note: this should be implemented for Git as well.