Update split i18n strings to be more translatable

What does this MR do and why?

Update the format of i18n strings that have multiple parts formatted into an English language sentence so that they can be translated into other languages that don't use similar syntax.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

This is a difficult set of changes to test in the GDK, as they range widely.

For most changes, the change is only a change in the copy the user sees and has no mechanical/functional change. These changes have already undergone Technical Writing review.

Mechanically, I think special reviewer scrutiny is probably due to my changes in app/helpers/emails_helper.rb and its spec file /spec/helpers/emails_helper_spec.rb. Previously, this file only returned the reason part of a split internationalized string, but I have changed it to return the entire message.

These changes can be most conveniently reviewed by visiting the mailer preview app (http://gdk.test:3000/rails/mailers) and checking the contents of generated emails.

Emails impacted by my changes:

• http://gdk.test:3000/rails/mailers/notify/approved_merge_request_email
• http://gdk.test:3000/rails/mailers/notify/closed_issue_email
• http://gdk.test:3000/rails/mailers/notify/closed_merge_request_email
• http://gdk.test:3000/rails/mailers/notify/issue_status_changed_email
• http://gdk.test:3000/rails/mailers/notify/merge_request_status_email
• http://gdk.test:3000/rails/mailers/notify/merge_when_pipeline_succeeds_email

Related to #450712 (closed), #444717 (closed)

app/helpers/emails_helper.rb

@@ -113,7 +113,7 @@ def closure_reason_text(closed_via, format: nil, name: nil)
 
       _("Issue was closed by %{name} via %{closed_via}") % { name: name, closed_via: closed_via }
     else
-      ""
+      _("Issue was closed by %{name}") % { name: name }
     end
   end