Add new deployment that builds the frontend on GitHub

[ezio-melotti]

Spin-off of #385 to try a different approach:

• build the frontend on GitHub
• copy the dist dir on the server with scp
• build the backend on the server

[ezio-melotti]

This seems to work now:

• the frontend is built on GitHub
• the dir is copied on the server with scp
• the backend is built on the server

The PR also includes some changes on the way the backend selects the right branch to build, that should make the switch more robust.