Make Unbricked into a submodule

[ISSOtm]

Pulling specific versions of its files via git2.

Fixes #110 (by keeping the variable even after outliving its usefulness, so maybe not a real fix)
Paves the way for #117, since each lesson can now link to a commit in a repo, and possibly #17 also.

Motivation

• The lessons were not always kept consistent between themselves, and it is particularly tedious to perform a “rebase” when something needs to be corrected in an earlier lesson.
• Allow a holistic view of the tutorial code (for Parts II and III) #117 forms the second half of the motivation, and tbh it's a big one.
• As a bonus, the .asm files are now served “inline”, instead of via GitHub. (This is not strictly necessary, but I think it makes more sense.)

Drawbacks

• Since unbricked is now a submodule, the associated submodule issues (git clone --recursive, git archive no longer working...) are brought in.
• This is even more work added onto the preprocessor; I am not super fond of the code I've written, but I'm not sure there's an elegant way to go about it either.
• Contributing to unbricked now requires PRing two separate repos (first the submodule, then this repo to update the commit being pointed at); and the former is not really a PR, since we need to keep one commit per lesson. OTOH it should reduce QA back-and-forth on PRs made to it.

Notes

I expect that the unbricked repo would be moved to the gbdev org before actually merging this.

I also expect to perform the same modifications to Galactic Armada afterwards, if this is accepted.