Rename v4 FontAwesome icons to match v5/v6 name

While FontAwesome provides aliases for all icons, we switch to the new names for consistency. See all changes at:
https://fontawesome.com/docs/web/setup/upgrade/upgrade-from-v4#icons-renamed-since-version-4

app/views/shared/_new_button.html.slim

@@ -2,5 +2,5 @@
   // default value for fetch will always be evaluated even if it is not returned
   - href_target = local_assigns.fetch(:path, false) || send(:"new_#{model.model_name.singular}_path")
   a.btn.btn-success href=href_target
-    i.fa.fa-plus
+    i.fa-solid.fa-plus
     = t('shared.new_model', model: model.model_name.human)