artists: allow members to rename, delete, and undelete artists.

Also allow Members to edit deleted or banned artists. Previously this
was limited in the html, but not in the backend.

app/views/artists/edit.html.erb

@@ -2,15 +2,10 @@
   <div id="a-edit">
     <h1>Edit Artist</h1>
 
-    <% if @artist.editable_by?(CurrentUser.user) %>
-      <%= render "form" %>
-
-    <% elsif !@artist.is_active? %>
-      <p>This artist entry is inactive and cannot be edited.</p>
-
-    <% elsif @artist.is_banned? %>
+    <% if @artist.is_banned? %>
       <p>The artist requested removal of this page.</p>
-
+    <% else %>
+      <%= render "form" %>
     <% end %>
   </div>
 </div>