artists: fix ban/unban actions.

Fix the ban! and unban! methods to:

* Lock the artist while it is being banned or unbanned.
* Perform the edits as a mass update, so that the posts are updated in parallel.
* Edit the artist as the banner rather than as the current user.
* Soft delete the banned_artist implication when an artist is unbanned instead of hard deleting it.
* Ignore the banned_artist implication if it's deleted.

test/unit/post_test.rb

@@ -384,11 +384,7 @@ class PostTest < ActiveSupport::TestCase
 
       context "with a banned artist" do
         setup do
-          CurrentUser.scoped(FactoryBot.create(:admin_user)) do
-            @artist = FactoryBot.create(:artist)
-            @artist.ban!
-            perform_enqueued_jobs
-          end
+          @artist = create(:artist, is_banned: true)
           @post = FactoryBot.create(:post, :tag_string => @artist.name)
         end