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/models/artist.rb

@@ -310,17 +310,26 @@ class Artist < ApplicationRecord
   end
 
   module FactoryMethods
+    # Make a new artist, fetching the defaults either from the given source, or
+    # from the source of the artist's last upload.
     def new_with_defaults(params)
-      Artist.new(params).tap do |artist|
-        if artist.name.present?
-          post = CurrentUser.without_safe_mode do
-            Post.tag_match("source:http #{artist.name}").where("true /* Artist.new_with_defaults */").first
-          end
-          unless post.nil? || post.source.blank?
-            artist.url_string = post.source
-          end
+      source = params.delete(:source)
+
+      if source.blank? && params[:name].present?
+        CurrentUser.without_safe_mode do
+          post = Post.tag_match("source:http* #{params[:name]}").first
+          source = post.try(:source)
         end
       end
+
+      if source.present?
+        artist = Sources::Strategies.find(source).new_artist
+        artist.attributes = params
+      else
+        artist = Artist.new(params)
+      end
+
+      artist.tap(&:validate) # run before_validation callbacks to normalize the names
     end
   end