[#72] Profile: Rating for Contributor (Delete Rating)

This MR resolves few things about issue #72 (closed):

• Implemented Delete Rating for Contributor on Katalog Kontributor page.
• Changed few things related to RatingContributor model, details on the MR changes tab.
• Changed few test related to RatingContributor model changes, details on the MR changes tab.
• Changed few things in RatingContributorForm, details on the MR changes tab.
• Fixed a little related to how Add/Update rating works, Add/Update Rating now requires user to login. This makes easier to Delete and/or Update the Ratings.
• Add/Update/Delete using the same view and method (i.e POST). Update and Delete invoked using AJAX from the html template with flag according to the requested method (Delete or Update).