Only show forks for users who can download code

The ForksController enforces this, so don't show the link if it won't go
anywhere.

CHANGELOG

@@ -32,6 +32,7 @@ v 8.8.0 (unreleased)
   - Backport GitHub Enterprise import support from EE
   - Create tags using Rugged for performance reasons. !3745
   - API: Expose Issue#user_notes_count. !3126 (Anton Popov)
+  - Don't show forks button when user can't view forks
   - Files over 5MB can only be viewed in their raw form, files over 1MB without highlighting !3718
   - Add support for supressing text diffs using .gitattributes on the default branch (Matt Oakes)
   - Add eager load paths to help prevent dependency load issues in Sidekiq workers. !3724

app/helpers/projects_helper.rb

@@ -138,10 +138,10 @@ module ProjectsHelper
   private
 
   def get_project_nav_tabs(project, current_user)
-    nav_tabs = [:home, :forks]
+    nav_tabs = [:home]
 
     if !project.empty_repo? && can?(current_user, :download_code, project)
-      nav_tabs << [:files, :commits, :network, :graphs]
+      nav_tabs << [:files, :commits, :network, :graphs, :forks]
     end
 
     if project.repo_exists? && can?(current_user, :read_merge_request, project)