Add an ability to bookmark contents of the Lesson

Lessons consist of multiple "content blocks", which can be a text section or a quiz (so far). It seems reasonable to me that students may want to mark some sections for future study/revisiting and be able to quickly navigate to those. They can do it already with browser bookmarks, but it would be better to have it built-in into the app itself for consistency.

It should be trivial to bookmark content blocks, and we can even implement an automatic bookmark for the last section you've viewed in the lesson to go with https://github.com/GDQuest/learn-gdscript/issues/43 and to address https://github.com/GDQuest/learn-gdscript/issues/39 in a graceful way.