Make GH jobs work with new branch workflow

[cewert]

Make github action jobs work with the new branch/release workflow.

Dev app will build:

• On push to master or *.*.z
• On all PRs

Prod app will build:

• On push to master or *.*.z
• On all PRs with a release-prep label

Changes

• Update workflows to use master branch instead of unstabl
• Create release-prep label
• Create release-prep.yml file
  • Trigger it on PR event labeled, opened, reopened, or synchronize
  • Only run when PR has the release-prep label