recast tests to refer to "app" instead of "link"

and "activate" instead of "link" as a verb, in messages,
test names, comments, variable names.

No functional change.
remove support for `link` artifact

* `app`, `suite`, and `artifact no longer implemented as
  synonyms for `link`
* backward-compatible code/comments removed
* creates class `Cask::Artifact::Suite`
* updates `brew cask info` to show specific artifact type
* fixes stray "Generic artifact" messages
Merge pull request #7065 from rolandwalker/remove_link_stanza

remove support for `link` artifact