added validations and unique index to allow users to fav projects only once

Been a long time since I worked on this! :smile:. Here's the thing I had promised to do here #224 :heart: 

Code to clear out duplicates:

``` ruby
FavoriteProject.all.group_by(&:user_id).each do |user, fav|
  fav.pop
  fav.each(&:destroy)
end
``` 

Hope this is fine! :)