Merge branch 'hover-descriptions' into 'master'

adds hover titles to merge request and issue pages

Fixes #5899

See merge request !2375

app/views/shared/issuable/_filter.html.haml

 .issues-filters
   .issues-state-filters
     %ul.center-top-menu
+      - if defined?(type) && type == :merge_requests
+        - page_context_word = 'merge requests'
+      - else
+        - page_context_word = 'issues'
       %li{class: ("active" if params[:state] == 'opened')}
-        = link_to page_filter_path(state: 'opened') do
+        = link_to page_filter_path(state: 'opened'), title: "Filter by #{page_context_word} that are currently opened." do
           #{state_filters_text_for(:opened, @project)}
 
       - if defined?(type) && type == :merge_requests
         %li{class: ("active" if params[:state] == 'merged')}
-          = link_to page_filter_path(state: 'merged') do
+          = link_to page_filter_path(state: 'merged'), title: 'Filter by merge requests that are currently merged.' do
             #{state_filters_text_for(:merged, @project)}
 
         %li{class: ("active" if params[:state] == 'closed')}
-          = link_to page_filter_path(state: 'closed') do
+          = link_to page_filter_path(state: 'closed'), title: 'Filter by merge requests that are currently closed and unmerged.' do
             #{state_filters_text_for(:closed, @project)}
       - else
         %li{class: ("active" if params[:state] == 'closed')}
-          = link_to page_filter_path(state: 'closed') do
+          = link_to page_filter_path(state: 'closed'), title: 'Filter by issues that are currently closed.' do
             #{state_filters_text_for(:closed, @project)}
 
       %li{class: ("active" if params[:state] == 'all')}
-        = link_to page_filter_path(state: 'all') do
+        = link_to page_filter_path(state: 'all'), title: "Show all #{page_context_word}." do
           #{state_filters_text_for(:all, @project)}
 
   .issues-details-filters.gray-content-block