Add an option to define keywords and a description per lesson

[Zerotask]

Is your feature request related to a problem? Please describe.
Currently there is just 1x keywords and description set in the app.html, which isn't very good for SEO.

Describe the solution you'd like
The solution depends on Svelte. It's probably something in the direction of named slots.

[Zerotask]

I'll wait on the response from sveltejs/kit#1540.
Working solutions are: using a root component (e. g. App.svelte) or (probably better) a store to optionally update the values from individual pages.

[Zerotask]

Currently you can set it with summary and tags, although it will produce double meta tags. I'll let this open until we found a better solution.