The route is `/lesson/<lesson-slug>`.
The level parameter is not yet taken into account.
The language is not yet taken into account either.

Babel fishes are awesome.
Using flags for languages is somewhat offensive,
since it's a gross, unfair simplification.
We'll remove the flags later.

We still need a proper flag for the rosetta stone.

This reverts commit 203dc39a for the README.

Perhaps we'll customize this further when I'll be inspired.

Implements #4

Also introduce a translations file for our "textbook" theme.

It is now submitted.

It's okay, PR is on the way.

(now I need to give it its own repository)
(and to translate it into french)

Notice that mathjax uses a CDN.
This is temporary.

This git tree is not going to be a bonzaï anytime soon.

(that was bound to happen sometime)
We're also adding two tentative CSS fixes for the theme and language-selector plugin.