Fix project index page

What does this MR do?

It allows user access to Project#index action which then redirect user either to projects page or explore page

Why was this MR needed?

Because now visiting /projects leads to 404 while based on the source code we want it to do redirect to correct page.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Marked the task Changelog entry added as completed

We have next code in projects controller but because of incorrect before_filter it never got executed

   def index
     redirect_to(current_user ? root_path : explore_root_path)
   end

Bug is found while working on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7121

Reassigned to @smcgivern

Marked the task All builds are passing as completed

Please use the new CHANGELOG process in future! https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/changelog.md

Status changed to merged

Mentioned in commit 034e8593

Good catch, @dzaporozhets!