Improve placeholder: search -> Search

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/layouts/_search.html.haml

 .search
   = form_tag search_path, method: :get, class: 'navbar-form pull-left' do |f|
-    = search_field_tag "search", nil, placeholder: 'search', class: "search-input form-control", spellcheck: false
+    = search_field_tag "search", nil, placeholder: 'Search', class: "search-input form-control", spellcheck: false
     = hidden_field_tag :group_id, @group.try(:id)
     - if @project && @project.persisted?
       = hidden_field_tag :project_id, @project.id