fix: don't render list bullet on CKEditor task lists

The content list-bullet rule (themes/default/scss/app.scss) draws a custom
`::before` bullet on `ul` and excludes `.contains-task-list` (markdown task
lists, where the checkbox is the marker). The CKEditor "Visual" editor emits
task lists with class `.todo-list`, which was not excluded, so the bullet
rendered in the task-list checkbox - in both the editor and the saved/rendered
output. Exclude `.todo-list` as well.

client/themes/default/scss/app.scss

@@ -663,7 +663,7 @@
     }
   }
 
-  ul:not(.tabset-tabs):not(.contains-task-list) {
+  ul:not(.tabset-tabs):not(.contains-task-list):not(.todo-list) {
     list-style: none;
     > li::before {
       position: absolute;