[IMP] website_forum: when edit the question at the remove the tags, that tags are now unlink, and tags are render properly on input field when edit the question.

Resolve Tags related bug,
  1. Tag now render properly when edit the question
  2. Removed old tag are unlink and add new tag work properly.

<b>Pad</b> : https://pad.odoo.com/p/r.9QsHXB1BzODehmWb
<b>Task</b> : https://www.odoo.com/web#id=10841&view_type=form&model=project.task&menu_id=3942&action=327

Thank you