Temporarily remove the primality lib submodule.

.gitmodules, visualizations/prime_spirals/js/primality
	-Remove submodule in preparation for directory renaming.
Move prime_spirals -> prime_number_spirals.

visualizations/(prime_spirals -> prime_number_spirals)/, .gitmodules
	-Rename `prime_spirals/` to `prime_number_spirals/`.
	-Re-add primality library submodule.
Merge branch 'move_prime_spirals' into develop

visualizations/(prime_spirals -> prime_number_spirals)/
	-Rename `prime_spirals/` to `prime_number_spirals/`.