Don’t allow adding a lesson to non-current sem/year

[blackmann]

Currently you can add lessons for sem 2 even though no school is in the second semester yet.

The “Add Lesson” button should be disabled when the selected sem/year is not the current.

Also show a message on the “Add lesson” page for inactive sem/year instead of the form. Something with the wording: “This year (xxx) and sem (xxx) is not ready for an entry” + some icon.

You can reference the current sem from client/res which is queried with values.get()(see: client/app/lib/values.ts)