Change flycheck initialization strategy

Initialize it globally and turn it off where needed, instead of enabling
it on demand. Also fixes void-function: flycheck-mode errors when
:feature syntax-checker is disabled. This is experimental.

Indirectly fixes #710

modules/lang/elm/config.el

@@ -21,6 +21,5 @@
 (def-package! flycheck-elm
   :when (featurep! :feature syntax-checker)
   :after elm-mode
-  :init (add-hook 'elm-mode-hook #'flycheck-mode)
   :config (add-to-list 'flycheck-checkers 'elm nil #'eq))