CSV export issues pluralization

Also fixed @issues_count in email after merge

app/views/notify/issues_csv_email.html.haml

 %p{ style: 'font-size:18px; text-align:center;  line-height:30px;' }
-  Your CSV export of #{ @issues_count } issues from project
+  Your CSV export of #{ pluralize(@written_count, 'issue') } from project
   %a{ href: project_url(@project), style: "color:#3777b0; text-decoration:none; display:block;" }
     = @project.full_name
   has been added to this email as an attachment.

app/views/notify/issues_csv_email.text.erb

-Your CSV export of <%= @written_count %> issues from project <%= @project.full_name %> (<%= project_url(@project) %>) has been added to this email as an attachment.
+Your CSV export of <%= pluralize(@written_count, 'issue') %> from project <%= @project.full_name %> (<%= project_url(@project) %>) has been added to this email as an attachment.
 
 <% if @truncated %>
 This attachment has been truncated to avoid exceeding a maximum allowed attachment size of 20MB. <%= @written_count %> of <%= @issues_count %> issues have been included. Consider re-exporting with a narrower selection of issues.

app/views/projects/issues/export_issues/_csv_download.html.haml

@@ -10,7 +10,7 @@
       .modal-header
         = icon('check', { class: 'export-checkmark' })
         %strong
-          #{cached_issuables_count_for_state(:issues, params[:state])} issues selected
+          #{pluralize(cached_issuables_count_for_state(:issues, params[:state]), 'issue')} selected
       .modal-body
         %div
           The CSV export will be created in the background. Once finished, it will be sent to