[IMP] website_forum : Karma check for tag creation and follow tag mechanism

Hello,

->  It will allow user to follow/unfollow Tag.
->  It will also check Tag case sensitivity and uniqueness.

More Details:
Task: https://www.odoo.com/web#id=8318&view_type=form&model=project.task&action=327
pad: https://pad.odoo.com/p/r.ePTaRKLxamjWaWAZ

Thanks
jas-odoo
Actually this is not the way karma retag should work. Retagging is the action of updating the tags on a question. So when writing on a question with tag_ids in the values, you should check for karma_retag. Creating tags should maybe be limited with anotker karma value (karma_tag_create). If no rights to create tags, disallow to create tags in the front-end and in the back-end.