Merge branch 'explore-nav' into 'master'

Remove icons from explore nav

## What does this MR do?
Removes icons from explore `nav/_explore`

## Why was this MR needed?
To match signed-in nav

## Screenshots (if relevant)
Before:   
![Screen_Shot_2016-07-20_at_6.39.01_AM](/uploads/9ccc11d151e37a0ef2fab681ccc07efa/Screen_Shot_2016-07-20_at_6.39.01_AM.png)

After:   
![Screen_Shot_2016-07-20_at_6.43.15_AM](/uploads/ae058b19cc3c887bccf6c221ac58cfc2/Screen_Shot_2016-07-20_at_6.43.15_AM.png)


See merge request !5362

app/views/layouts/nav/_explore.html.haml

 %ul.nav.nav-sidebar
   = nav_link(path: ['dashboard#show', 'root#show', 'projects#trending', 'projects#starred', 'projects#index'], html_options: {class: 'home'}) do
     = link_to explore_root_path, title: 'Projects' do
-      = icon('bookmark fw')
       %span
         Projects
   = nav_link(controller: [:groups, 'groups/milestones', 'groups/group_members']) do
     = link_to explore_groups_path, title: 'Groups' do
-      = icon('group fw')
       %span
         Groups
   = nav_link(controller: :snippets) do
     = link_to explore_snippets_path, title: 'Snippets' do
-      = icon('clipboard fw')
       %span
         Snippets
   = nav_link(controller: :help) do
     = link_to help_path, title: 'Help' do
-      = icon('question-circle fw')
       %span
         Help