Recast various uses of "link"

The term "link" is inherently imprecise and has been used somewhat roughly at times, referring to
* URLs
* symlinks and hardlinks
* the action of creating links into the staging directory
* the name of an artifact (and associated stanza)
* Alfred activation

This PR is concerned with using the term "URL" instead of "link".  Elsewhere, we
are moving toward the term "activate" for the action of creating links, as well as
removing the `link` stanza/artifact entirely.

It would also be preferable to substitute "symlink" for "link" in docs and messages
where that is the meaning.

`brew cask alfred link` has not been altered, but we should consider doing so for
clarity.