artists: fix editing invalid urls in artist entries (fix #3720, #3927, #3781)

Convert to an autosave association on urls. This ensures that when we
save the artist we only validate the added urls, not bad urls that we're
trying to remove, and that url validation errors are propagated up to
the artist object.

This also fixes invalid urls being saved in the artist history despite
validation failing (#3720).

app/models/artist_url.rb

@@ -1,5 +1,4 @@
 class ArtistUrl < ApplicationRecord
-  before_validation :parse_prefix
   before_validation :initialize_normalized_url, on: :create
   before_validation :normalize
   validates :url, presence: true, uniqueness: { scope: :artist_id }
@@ -9,12 +8,11 @@ class ArtistUrl < ApplicationRecord
   scope :url_matches, ->(url) { url_attribute_matches(:url, url) }
   scope :normalized_url_matches, ->(url) { url_attribute_matches(:normalized_url, url) }
 
-  def self.strip_prefixes(url)
-    url.sub(/^[-]+/, "")
-  end
+  def self.parse_prefix(url)
+    prefix, url = url.match(/\A(-)?(.*)/)[1, 2]
+    is_active = prefix.nil?
 
-  def self.is_active?(url)
-    url !~ /^-/
+    [is_active, url]
   end
 
   def self.normalize(url)
@@ -88,14 +86,6 @@ class ArtistUrl < ApplicationRecord
     end
   end
 
-  def parse_prefix
-    case url
-    when /^-/
-      self.url = url[1..-1]
-      self.is_active = false
-    end
-  end
-
   def priority
     if normalized_url =~ /pixiv\.net\/member\.php/
       10
@@ -132,8 +122,8 @@ class ArtistUrl < ApplicationRecord
 
   def validate_url_format
     uri = Addressable::URI.parse(url)
-    errors[:url] << "#{uri} must begin with http:// or https://" if !uri.scheme.in?(%w[http https])
+    errors[:url] << "'#{uri}' must begin with http:// or https:// " if !uri.scheme.in?(%w[http https])
   rescue Addressable::URI::InvalidURIError => error
-    errors[:url] << "is malformed: #{error}"
+    errors[:url] << "'#{uri}' is malformed: #{error}"
   end
 end