Simplify lesson content blocks

[NathanLovato]

The UI content block scene currently has a node hierarchy that's more complex than needed. This is most likely what creates this little hiccup when you click a lesson and it gets populated.

It should be simplified to only create the necessary nodes.

This is something worth doing after #1188 because then the parser would be able to build a minimal structure like only adding box containers when the parsed BBCode/document structure requires it.

[NathanLovato]

@Razoric480 I assigned you this as part of a set of next tickets to pick from. In Godot 3 lesson content block resources were the base model to add anything to a lesson, we had something generic to represent a text paragraph, a visual, etc.

With this task the goal would be to get rid of this general purpose lesson block and instead, e.g. for a series of contiguous text paragraphs, spawn a single RichTextLabel node. For a visual, just spawn that visual, etc. based on the parsed BBCode AST.

This could lighten the lessons a bit, though I don't know by how much. I'm curious if it makes a difference for #1265 (not sure still if it's the amount of text, number of controls total in the tree, etc. that cause this freeze).

Let me know if you need more information!