[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
you can use `Warning` in the `openerp.exceptions`