Rename the "Classic" theme to "Shoelace"

[LeaVerou]

This was discussed in the strategy call last week. @claviska agreed that "Classic" is a bit misleading; it sounds like "classic (i.e. timeless) style" where it actually means Shoelace. We can't use Shoelace for legacy reasons, so perhaps "Legacy" might be a better name? We generally want to discourage use of this in new projects, since it mainly exists to make onboarding Shoelace users easier.

[claviska]

I agree that "Classic" is misleading, but "Legacy" carries a negative connotation. After a casual discussion with @davegandy, we feel that renaming it to "Shoelace" is a nice hat tip to the project's roots and shouldn't be a problem legally or technically.

So let's rename it to "Shoelace" before the alpha release.

[claviska]

Fixed in #959