Fix importers with OCC

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/1180

Using validates_uniqueness_of does not guarantee the absence of duplicate record insertions. When users click button "import all" it will create duplicating group in database and data integrity will be broken.

http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of

I use optimistic concurrency control for handling this situation.

See merge request !1647

Conflicts:
	db/schema.rb