Added author username tooltip in issuable header

Closes #13771

app/helpers/issuables_helper.rb

@@ -61,7 +61,7 @@ module IssuablesHelper
     output = content_tag :strong, "#{text} #{issuable.to_reference}", class: "identifier"
     output << " opened #{time_ago_with_tooltip(issuable.created_at)} by ".html_safe
     output << content_tag(:strong) do
-      author_output = link_to_member(project, issuable.author, size: 24, mobile_classes: "hidden-xs")
+      author_output = link_to_member(project, issuable.author, size: 24, mobile_classes: "hidden-xs", tooltip: true)
       author_output << link_to_member(project, issuable.author, size: 24, by_username: true, avatar: false, mobile_classes: "hidden-sm hidden-md hidden-lg")
     end
   end

app/helpers/projects_helper.rb

@@ -19,7 +19,7 @@ module ProjectsHelper
   end
 
   def link_to_member(project, author, opts = {}, &block)
-    default_opts = { avatar: true, name: true, size: 16, author_class: 'author', title: ":name" }
+    default_opts = { avatar: true, name: true, size: 16, author_class: 'author', title: ":name", tooltip: false }
     opts = default_opts.merge(opts)
 
     return "(deleted)" unless author
@@ -33,7 +33,9 @@ module ProjectsHelper
     if opts[:by_username]
       author_html << content_tag(:span, sanitize("@#{author.username}"), class: opts[:author_class]) if opts[:name]
     else
-      author_html << content_tag(:span, sanitize(author.name), class: opts[:author_class]) if opts[:name]
+      tooltip_data = { placement: 'top' }
+      author_html << content_tag(:span, sanitize(author.name), class: [opts[:author_class], ('has-tooltip' if opts[:tooltip])],
+        title: (author.to_reference if opts[:tooltip]), data: (tooltip_data if opts[:tooltip])) if opts[:name]
     end
 
     author_html << capture(&block) if block