artists: rename is_active flag to is_deleted.

Rename is_active to is_deleted. This is for better consistency with
other models, and to reduce confusion over what "active" means for
artists. Sometimes users think active is for whether the artist is
actively producing work.

app/controllers/artists_controller.rb

@@ -55,7 +55,7 @@ class ArtistsController < ApplicationController
   end
 
   def destroy
-    @artist.update_attribute(:is_active, false)
+    @artist.update_attribute(:is_deleted, true)
     redirect_to(artist_path(@artist), :notice => "Artist deleted")
   end
 
@@ -94,7 +94,7 @@ class ArtistsController < ApplicationController
   end
 
   def artist_params(context = nil)
-    permitted_params = %i[name other_names other_names_string group_name url_string notes is_active]
+    permitted_params = %i[name other_names other_names_string group_name url_string notes is_deleted]
     permitted_params << { wiki_page_attributes: %i[id body] }
     permitted_params << :source if context == :new