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.

test/functional/artists_controller_test.rb

@@ -127,7 +127,7 @@ class ArtistsControllerTest < ActionDispatch::IntegrationTest
     should "create an artist" do
       attributes = FactoryBot.attributes_for(:artist)
       assert_difference("Artist.count", 1) do
-        attributes.delete(:is_active)
+        attributes.delete(:is_deleted)
         post_auth artists_path, @user, params: {artist: attributes}
       end
       artist = Artist.find_by_name(attributes[:name])
@@ -171,14 +171,14 @@ class ArtistsControllerTest < ActionDispatch::IntegrationTest
       delete_auth artist_path(@artist.id), @builder
       assert_redirected_to(artist_path(@artist.id))
       @artist.reload
-      assert_equal(false, @artist.is_active)
+      assert_equal(true, @artist.is_deleted)
     end
 
     should "undelete an artist" do
       @builder = create(:builder_user)
-      put_auth artist_path(@artist.id), @builder, params: {artist: {is_active: true}}
+      put_auth artist_path(@artist.id), @builder, params: {artist: {is_deleted: false}}
       assert_redirected_to(artist_path(@artist.id))
-      assert_equal(true, @artist.reload.is_active)
+      assert_equal(false, @artist.reload.is_deleted)
     end
 
     context "reverting an artist" do