artists: improve prefilling of new artist form (#4028)

* When creating an artist by clicking the '?' next to the artist tag in
  the tag list, prefill the new artist form by finding the artist's last
  upload and fetching its source data.

  Previously we filled the urls with the source of the artist's last
  upload, which was wrong because it was usually a direct image URL (#3078).

* Fix the other names field not escaping spaces within names to underscores.

* Fix the other names field being potentially prefilled with duplicate names.

app/controllers/artists_controller.rb

@@ -6,7 +6,7 @@ class ArtistsController < ApplicationController
   before_action :load_artist, :only => [:ban, :unban, :show, :edit, :update, :destroy, :undelete]
 
   def new
-    @artist = Artist.new_with_defaults(artist_params)
+    @artist = Artist.new_with_defaults(artist_params(:new))
     respond_with(@artist)
   end
 
@@ -104,9 +104,10 @@ private
     sp.permit!
   end
 
-  def artist_params
+  def artist_params(context = nil)
     permitted_params = %i[name other_names other_names_string group_name url_string notes]
     permitted_params << :is_active if CurrentUser.is_builder?
+    permitted_params << :source if context == :new
 
     params.fetch(:artist, {}).permit(permitted_params)
   end