Modify project structure

Before, we had shared/shared because the cross project was inside the shared folder, which doesn't make a lot of sense.

I moved the cross-project at the root and cleaned up build.sbt to be consistent with course-material/labs/webapp-rps

Basically:

1. server/ becomes jvm/
2. client/ becomes js/
3. shared/shared becomes shared/
4. shared/js disappears (they were empty so not pushed on Gitlab but they do exist)
5. shared/jvm disappears (they were empty so not pushed on Gitlab but they do exist)
6. build.sbt gets greatly simplified
7. Removed unused plugin sbt-jsdependencies

On the RPS side, almost nothing changes