Improve projects controller specs

The following discussion from gitlab-ce!12840 should be addressed:

• @grzesiek started a discussion: (+1 comment)

  There some technical debt in these specs. We set @project instance variable in the controller manually, using set_instance_variable. This means that we override assigning @project in project method defined in Projects::ApplicationController. The result is that we are using incorrect parameters in most of the specs in this file - namespace_id: project.namespace and id: project.id instead of using paths.

  I will resolve this comment by creating a follow-up gitlab-ce~3011693 issue.

/cc @smcgivern