[ticket/12976] Correctly use plurals for attachment count in UCP navigation

PHPBB3-12976
Merge pull request #2936 from nickvergessen/ticket/12976

[ticket/12976] Correctly use plurals for attachment count in UCP navigation

* nickvergessen/ticket/12976:
  [ticket/12976] Correctly use plurals for attachment count in UCP navigation
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12976] Correctly use plurals for attachment count in UCP navigation

Conflicts:
	phpBB/styles/subsilver2/template/ucp_attachments.html