dashboard/app: gcloud app deploy costs ~6 minutes

[tarasmadan]

Is your feature request related to a problem? Please describe.
I often test UI in the AppEngine env.

Describe the solution you'd like
https://stackoverflow.com/a/40129720 claims we can x2 the speed reusing the version.

For every PR we can create the temporary version.
gcloud app deploy app.yaml --version=hash($PR_number + $salt).
After the PR merge, this test version should be removed.