Add "Copy to clipboard" buttons

Adds buttons to the commit list SHAs and the cross-project reference
data on issuables.

app/helpers/clipboard_helper.rb

+module ClipboardHelper
+  def clipboard_button(target = nil)
+    content_tag :button,
+      icon('clipboard'),
+      class: 'btn btn-xs btn-clipboard js-clipboard-trigger',
+      type: :button
+  end
+end

app/views/projects/commits/_commit.html.haml

@@ -21,7 +21,8 @@
           = link_to ci_status_path(ci_commit), class: "c#{ci_status_color(ci_commit)}" do
             = ci_status_icon(ci_commit)
            
-        = link_to commit.short_id, namespace_project_commit_path(project.namespace, project, commit), class: "commit_short_id"
+        = clipboard_button
+        = link_to commit.short_id, namespace_project_commit_path(project.namespace, project, commit), class: "commit_short_id", data: {clipboard_text: commit.id}
       .notes_count
         - if note_count > 0

app/views/projects/issues/_discussion.html.haml

@@ -17,8 +17,10 @@
         - @participants.each do |participant|
           = link_to_member(@project, participant, name: false, size: 24)
     .col-md-3
-      %span.slead.has_tooltip{title: 'Cross-project reference'}
-        = cross_project_reference(@project, @issue)
+      .input-group.cross-project-reference
+        %span.slead.has_tooltip{title: 'Cross-project reference'}
+          = cross_project_reference(@project, @issue)
+        = clipboard_button
 .row
   %section.col-md-9