Editorial: Define+use list-concatenation

[jmdyck]

It's analogous to string-concatenation.

I've made separate commits for ease of reviewing. You could squash them all except perhaps the last.

[jmdyck]

(force-pushed to rebase to master, and handle a couple cases added by #1668)

[michaelficarra]

In cases where we are introducing new numbered aliases, I would prefer we number the first one as well (as in, names1) to better convey that it is just another part, not the whole. LGTM otherwise.

[jmdyck]

In cases where we are introducing new numbered aliases, I would prefer we number the first one as well (as in, names1) ...

That'd be my preference as well, but I left it out to minimize the diff.