[codeowners] allow overrides to take higher precedence (#150821)

With #149344 the codeowners has many more entries in it, and these
entries are taking precedence because in codeowner files:

https://help.github.com/articles/about-codeowners/
> Order is important; the last matching pattern takes the most
precedence.

This reverses the order of the codeowners file, placing all of the
auto-generated codeowners at the top of the file, followed by the
manually managed overrides, and finally followed by the "ultimate
priority" rules which gives design codeowners over all .scss files in
the repo.