There are now 3 modes for deployment: next, stable, archive.
We compute which mode (and other deployment properties)
from the `TRAVIS_BRANCH` and the `STABLE_BRANCH`.

If the TRAVIS_BRANCH is master we deploy as "next".
Otherwise if the branch is the highest of its minor versions
we deploy as "stable" if the `TRAVIS_BRANCH` matches the `STABLE_BRANCH` or
else "archive".

For "archive" deployments we compute the firebase project and deployment
url based on the major version of the `TRAVIS_BRANCH`.

As well as choosing where to deploy the build, we also use this
to select the environment file for the AIO Angular app.
This will enable the app to change its rendering and behaviour
based on its mode.

See #18287