Support stable and unstable versions of the explorer

[jpraynaud]

Why

We want to have 2 versions of the explorers: one for production use and another for testing unstable features.

What

Build and deploy 2 versions of the explorer, a stable and an unstable.

How

• Build and deploy the unstable version of the explorer with the CI workflow (/explorer-unstable)
• Deploy the stable version of the explorer with the CI workflow (/explorer)
• Build the release version of the explorer with the Pre-Release workflow