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.

app/controllers/artists_controller.rb

@@ -19,13 +19,13 @@ class ArtistsController < ApplicationController
 
   def ban
     @artist = authorize Artist.find(params[:id])
-    @artist.ban!(banner: CurrentUser.user)
+    @artist.ban!(CurrentUser.user)
     redirect_to(artist_path(@artist), :notice => "Artist was banned")
   end
 
   def unban
     @artist = authorize Artist.find(params[:id])
-    @artist.unban!
+    @artist.unban!(CurrentUser.user)
     redirect_to(artist_path(@artist), :notice => "Artist was unbanned")
   end