artists: convert other_names to array (#3987).

app/views/artists/_form.html.erb

@@ -14,7 +14,7 @@
       <p><%= @artist.name %></p>
     <% end %>
   </div>
-  <%= f.input :other_names_comma, :hint => "Separate with commas", :as => :text, :label => "Other names" %>
+  <%= f.input :other_names_string, :hint => "Separate with spaces", :as => :text, :label => "Other names" %>
   <%= f.input :group_name %>
   <%= f.input :url_string, :label => "URLs", :as => :text, :input_html => {:size => "50x5", :value => params.dig(:artist, :url_string) || @artist.urls.join("\n")}, :hint => "You can prefix a URL with - to mark it as dead." %>